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diff --git a/guides/assets/images/association_basics/polymorphic.png b/guides/assets/images/association_basics/polymorphic.png
index 53f527f6e36b9..90fe678edff0a 100644
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diff --git a/guides/source/active_record_composite_primary_keys.md b/guides/source/active_record_composite_primary_keys.md
index 59b5453c2f56c..bb2464c0e4aa9 100644
--- a/guides/source/active_record_composite_primary_keys.md
+++ b/guides/source/active_record_composite_primary_keys.md
@@ -123,9 +123,14 @@ guide to learn more.
Associations between Models with Composite Primary Keys
-------------------------------------------------------
-Rails is often able to infer the primary key - foreign key information between
-associated models with composite primary keys without needing extra information.
-Take the following example:
+Rails can often infer the primary key-foreign key relationships between
+associated models. However, when dealing with composite primary keys, Rails
+typically defaults to using only part of the composite key, usually the `id`
+column, unless explicitly instructed otherwise. This default behavior only works
+if the model's composite primary key contains the `:id` column, _and_ the column
+is unique for all records.
+
+Consider the following example:
```ruby
class Order < ApplicationRecord
@@ -138,29 +143,40 @@ class Book < ApplicationRecord
end
```
-Here, Rails assumes that the `:id` column should be used as the primary key for
-the association between an order and its books, just as with a regular
-`has_many` / `belongs_to` association. It will infer that the foreign key column
-on the `books` table is `:order_id`. Accessing a book's order:
+In this setup, `Order` has a composite primary key consisting of `[:shop_id,
+:id]`, and `Book` belongs to `Order`. Rails will assume that the `:id` column
+should be used as the primary key for the association between an order and its
+books. It will infer that the foreign key column on the books table is
+`:order_id`.
+
+Below we create an `Order` and a `Book` associated with it:
```ruby
order = Order.create!(id: [1, 2], status: "pending")
book = order.books.create!(title: "A Cool Book")
+```
+To access the book's order, we reload the association:
+
+```ruby
book.reload.order
```
-will generate the following SQL to access the order:
+When doing so, Rails will generate the following SQL to access the order:
```sql
SELECT * FROM orders WHERE id = 2
```
-This only works if the model's composite primary key contains the `:id` column,
-_and_ the column is unique for all records. In order to use the full composite
-primary key in associations, set the `foreign_key:` option on the
-association. This option specifies a composite foreign key on the association,
-meaning that all columns in the foreign key will be used to query the
+You can see that Rails uses the order's `id` in its query, rather than both the
+`shop_id` and the `id`. In this case, the `id` is sufficient because the model's
+composite primary key does in fact contain the `:id` column, _and_ the column is
+unique for all records.
+
+However, if the above requirements are not met or you would like to use the full
+composite primary key in associations, you can set the `foreign_key:` option on
+the association. This option specifies a composite foreign key on the
+association; all columns in the foreign key will be used when querying the
associated record(s). For example:
```ruby
@@ -174,16 +190,25 @@ class Book < ApplicationRecord
end
```
-Accessing a book's author:
+In this setup, `Author` has a composite primary key consisting of `[:first_name,
+:last_name]`, and `Book` belongs to `Author` with a composite foreign key
+`[:author_first_name, :author_last_name]`.
+
+Create an `Author` and a `Book` associated with it:
```ruby
author = Author.create!(first_name: "Jane", last_name: "Doe")
-book = author.books.create!(title: "A Cool Book")
+book = author.books.create!(title: "A Cool Book", author_first_name: "Jane", author_last_name: "Doe")
+```
+To access the book's author, we reload the association:
+
+```ruby
book.reload.author
```
-will use `:first_name` _and_ `:last_name` in the SQL query:
+Rails will now use the `:first_name` _and_ `:last_name` from the composite
+primary key in the SQL query:
```sql
SELECT * FROM authors WHERE first_name = 'Jane' AND last_name = 'Doe'
diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md
index fcc34917b4d37..c5e55f83e784b 100644
--- a/guides/source/active_record_migrations.md
+++ b/guides/source/active_record_migrations.md
@@ -559,9 +559,19 @@ create_join_table :products, :categories, column_options: { null: true }
```
By default, the name of the join table comes from the union of the first two
-arguments provided to create_join_table, in alphabetical order. In this case,
+arguments provided to create_join_table, in lexical order. In this case,
the table would be named `categories_products`.
+WARNING: The precedence between model names is calculated using the `<=>`
+operator for `String`. This means that if the strings are of different lengths,
+and the strings are equal when compared up to the shortest length, then the
+longer string is considered of higher lexical precedence than the shorter one.
+For example, one would expect the tables "paper_boxes" and "papers" to generate
+a join table name of "papers_paper_boxes" because of the length of the name
+"paper_boxes", but it in fact generates a join table name of
+"paper_boxes_papers" (because the underscore '\_' is lexicographically _less_
+than 's' in common encodings).
+
To customize the name of the table, provide a `:table_name` option:
```ruby
diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md
index 13a7ab4fae800..61b73563302c1 100644
--- a/guides/source/association_basics.md
+++ b/guides/source/association_basics.md
@@ -7,20 +7,58 @@ This guide covers the association features of Active Record.
After reading this guide, you will know how to:
+* Understand the various types of associations.
* Declare associations between Active Record models.
-* Understand the various types of Active Record associations.
-* Use the methods added to your models by creating associations.
+* Choose the right association type for your models.
+* Use Single Table Inheritance.
+* Setting up and using Delegated Types.
--------------------------------------------------------------------------------
-Why Associations?
+Associations Overview
-----------------
-In Rails, an _association_ is a connection between two Active Record models. Why do we need associations between models? Because they make common operations simpler and easier in your code.
+Active Record associations allow you to define relationships between models.
+_Associations_ are implemented as special macro style calls that make it easy to
+tell Rails how your models relate to each other, which helps you manage your
+data more effectively, and makes common operations simpler and easier to read.
-For example, consider a simple Rails application that includes a model for authors and a model for books. Each author can have many books.
+INFO: A macro-style call is a method that generates or modifies other methods at
+runtime, allowing for concise and expressive declarations of functionality, such
+as defining model associations in Rails. For example, `has_many :comments`.
-Without associations, the model declarations would look like this:
+When you set up an association, Rails helps define and manage the [Primary
+Key](https://en.wikipedia.org/wiki/Primary_key) and [Foreign
+Key](https://en.wikipedia.org/wiki/Foreign_key) relationships between instances
+of the two models, while the database ensures that your data stays consistent
+and properly linked.
+
+This makes it easy to keep track of which records are related. It also adds
+useful methods to your models so you can work with related data more easily.
+
+Consider a simple Rails application with models for authors and books.
+
+### Without Associations
+
+Without associations, creating and deleting books for that author would require
+a tedious and manual process. Here's what that would look like:
+
+```ruby
+class CreateAuthors < ActiveRecord::Migration[7.2]
+ def change
+ create_table :authors do |t|
+ t.string :name
+ t.timestamps
+ end
+
+ create_table :books do |t|
+ t.references :author
+ t.datetime :published_at
+ t.timestamps
+ end
+ end
+end
+```
```ruby
class Author < ApplicationRecord
@@ -30,13 +68,16 @@ class Book < ApplicationRecord
end
```
-Now, suppose we wanted to add a new book for an existing author. We'd need to do something like this:
+To add a new book for an existing author, you'd need to provide the `author_id`
+value when creating the book.
```ruby
-@book = Book.create(published_at: Time.now, author_id: @author.id)
+@book = Book.create(author_id: @author.id, published_at: Time.now)
```
-Or consider deleting an author, and ensuring that all of its books get deleted as well:
+To delete an author and ensure all their books are also deleted, you need to
+retrieve all the author's `books`, loop through each `book` to destroy it, and
+then destroy the author.
```ruby
@books = Book.where(author_id: @author.id)
@@ -46,7 +87,12 @@ end
@author.destroy
```
-With Active Record associations, we can streamline these - and other - operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up authors and books:
+### Using Associations
+
+However, with associations, we can streamline these operations, as well as
+others, by explicitly informing Rails about the relationship between the two
+models. Here's the revised code for setting up authors and books using
+associations:
```ruby
class Author < ApplicationRecord
@@ -58,26 +104,48 @@ class Book < ApplicationRecord
end
```
-With this change, creating a new book for a particular author is easier:
+With this change, creating a new book for a particular author is simpler:
```ruby
@book = @author.books.create(published_at: Time.now)
```
-Deleting an author and all of its books is *much* easier:
+Deleting an author and all of its books is much easier:
```ruby
@author.destroy
```
-To learn more about the different types of associations, read the next section of this guide. That's followed by some tips and tricks for working with associations, and then by a complete reference to the methods and options for associations in Rails.
+When you set up an association in Rails, you still need to create a
+[migration](active_record_migrations.html) to ensure that the database is
+properly configured to handle the association. This migration will need to add
+the necessary foreign key columns to your database tables.
-The Types of Associations
--------------------------
+For example, if you set up a `belongs_to :author` association in the `Book`
+model, you would create a migration to add the `author_id` column to the `books`
+table:
+
+```bash
+rails generate migration AddAuthorToBooks author:references
+```
+
+This migration will add the `author_id` column and set up the foreign key
+relationship in the database, ensuring that your models and database stay in
+sync.
+
+To learn more about the different types of associations, you can read the next
+section of this guide. Following that, you'll find some tips and tricks for
+working with associations. Finally, there's a complete reference to the methods
+and options for associations in Rails.
-Rails supports six types of associations, each with a particular use-case in mind.
+Types of Associations
+---------------------
-Here is a list of all of the supported types with a link to their API docs for more detailed information on how to use them, their method parameters, etc.
+Rails supports six types of associations, each with a particular use-case in
+mind.
+
+Here is a list of all of the supported types with a link to their API docs for
+more detailed information on how to use them, their method parameters, etc.
* [`belongs_to`][]
* [`has_one`][]
@@ -86,18 +154,26 @@ Here is a list of all of the supported types with a link to their API docs for m
* [`has_one :through`][`has_one`]
* [`has_and_belongs_to_many`][]
-Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model `belongs_to` another, you instruct Rails to maintain [Primary Key](https://en.wikipedia.org/wiki/Primary_key)-[Foreign Key](https://en.wikipedia.org/wiki/Foreign_key) information between instances of the two models, and you also get a number of utility methods added to your model.
-
-In the remainder of this guide, you'll learn how to declare and use the various forms of associations. But first, a quick introduction to the situations where each association type is appropriate.
+In the remainder of this guide, you'll learn how to declare and use the various
+forms of associations. First, let's take a quick look at the situations where
+each association type is appropriate.
-[`belongs_to`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to
-[`has_and_belongs_to_many`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many
-[`has_many`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many
-[`has_one`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_one
+[`belongs_to`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to
+[`has_and_belongs_to_many`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many
+[`has_many`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many
+[`has_one`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_one
-### The `belongs_to` Association
+### `belongs_to`
-A [`belongs_to`][] association sets up a connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model. For example, if your application includes authors and books, and each book can be assigned to exactly one author, you'd declare the book model this way:
+A [`belongs_to`][] association sets up a relationship with another model, such
+that each instance of the declaring model "belongs to" one instance of the other
+model. For example, if your application includes authors and books, and each
+book can be assigned to exactly one author, you'd declare the book model this
+way:
```ruby
class Book < ApplicationRecord
@@ -107,7 +183,12 @@ end
-NOTE: `belongs_to` associations _must_ use the singular term. If you used the pluralized form in the above example for the `author` association in the `Book` model and tried to create the instance by `Book.create(authors: @author)`, you would be told that there was an "uninitialized constant Book::Authors". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too.
+NOTE: A `belongs_to` association _must_ use the singular term. If you use the
+plural form, like `belongs_to :authors` in the `Book` model, and try to create a
+book with `Book.create(authors: @author)`, Rails will give you an "uninitialized
+constant Book::Authors" error. This happens because Rails automatically infers
+the class name from the association name. If the association name is `:authors`,
+Rails will look for a class named `Authors` instead of `Author`.
The corresponding migration might look like this:
@@ -128,222 +209,233 @@ class CreateBooks < ActiveRecord::Migration[7.2]
end
```
-When used alone, `belongs_to` produces a one-directional one-to-one connection. Therefore each book in the above example "knows" its author, but the authors don't know about their books.
-To setup a [bi-directional association](#bi-directional-associations) - use `belongs_to` in combination with a `has_one` or `has_many` on the other model, in this case the Author model.
+In database terms, the `belongs_to` association says that this model's table
+contains a column which represents a reference to another table. This can be
+used to set up one-to-one or one-to-many relations, depending on the setup. If
+the table _of the other class_ contains the reference in a one-to-one relation,
+then you should use `has_one` instead.
-`belongs_to` does not ensure reference consistency if `optional` is set to true, so depending on the use case, you might also need to add a database-level foreign key constraint on the reference column, like this:
+When used alone, `belongs_to` produces a one-directional one-to-one
+relationship. Therefore each book in the above example "knows" its author, but
+the authors don't know about their books. To setup a [bi-directional
+association](#bi-directional-associations) - use `belongs_to` in combination
+with a `has_one` or `has_many` on the other model, in this case the Author
+model.
+
+NOTE: By default `belongs_to` validates the presence of the associated record to
+guarantee reference consistency.
If `optional` is set to true in the
+model, then `belongs_to` does not guarantee reference consistency. This means
+that the foreign key in one table might not reliably point to a valid primary
+key in the referenced table.
```ruby
-create_table :books do |t|
- t.belongs_to :author, foreign_key: true
- # ...
+class Book < ApplicationRecord
+ belongs_to :author, optional: true
end
```
-### The `has_one` Association
-
-A [`has_one`][] association indicates that one other model has a reference to this model. That model can be fetched through this association.
-
-For example, if each supplier in your application has only one account, you'd declare the supplier model like this:
+Hence, depending on the use case, you might also need to add a database-level
+foreign key constraint on the reference column, like this:
```ruby
-class Supplier < ApplicationRecord
- has_one :account
+create_table :books do |t|
+ t.belongs_to :author, foreign_key: true
+ # ...
end
```
-The main difference from `belongs_to` is that the link column `supplier_id` is located in the other table:
+This ensures that even though `optional: true` allows `author_id` to be NULL,
+when it's not NULL, it must still reference a valid record in the authors table.
-
+#### Methods Added by `belongs_to`
-The corresponding migration might look like this:
+When you declare a `belongs_to` association, the declaring class automatically
+gains numerous methods related to the association. Some of these are:
-```ruby
-class CreateSuppliers < ActiveRecord::Migration[7.2]
- def change
- create_table :suppliers do |t|
- t.string :name
- t.timestamps
- end
+* `association=(associate)`
+* `build_association(attributes = {})`
+* `create_association(attributes = {})`
+* `create_association!(attributes = {})`
+* `reload_association`
+* `reset_association`
+* `association_changed?`
+* `association_previously_changed?`
- create_table :accounts do |t|
- t.belongs_to :supplier
- t.string :account_number
- t.timestamps
- end
- end
-end
-```
+We'll discuss some of the common methods, but you can find an exhaustive list in
+the [ActiveRecord Associations
+API](https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to).
-Depending on the use case, you might also need to create a unique index and/or
-a foreign key constraint on the supplier column for the accounts table. In this
-case, the column definition might look like this:
+In all of the above methods, `association` is replaced with the symbol passed as
+the first argument to `belongs_to`. For example, given the declaration:
```ruby
-create_table :accounts do |t|
- t.belongs_to :supplier, index: { unique: true }, foreign_key: true
- # ...
+# app/models/book.rb
+class Book < ApplicationRecord
+ belongs_to :author
end
-```
-
-This relation can be [bi-directional](#bi-directional-associations) when used in combination with `belongs_to` on the other model.
-
-### The `has_many` Association
-A [`has_many`][] association is similar to `has_one`, but indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a `belongs_to` association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing authors and books, the author model could be declared like this:
-
-```ruby
+# app/models/author.rb
class Author < ApplicationRecord
has_many :books
+ validates :name, presence: true
end
```
-NOTE: The name of the other model is pluralized when declaring a `has_many` association.
+An instance of the `Book` model will have the following methods:
-
+* `author`
+* `author=`
+* `build_author`
+* `create_author`
+* `create_author!`
+* `reload_author`
+* `reset_author`
+* `author_changed?`
+* `author_previously_changed?`
-The corresponding migration might look like this:
+NOTE: When initializing a new `has_one` or `belongs_to` association you must use
+the `build_` prefix to build the association, rather than the
+`association.build` method that would be used for `has_many` or
+`has_and_belongs_to_many` associations. To create one, use the `create_` prefix.
-```ruby
-class CreateAuthors < ActiveRecord::Migration[7.2]
- def change
- create_table :authors do |t|
- t.string :name
- t.timestamps
- end
+##### Retrieving the association
- create_table :books do |t|
- t.belongs_to :author
- t.datetime :published_at
- t.timestamps
- end
- end
-end
+The `association` method returns the associated object, if any. If no associated
+object is found, it returns `nil`.
+
+```ruby
+@author = @book.author
```
-Depending on the use case, it's usually a good idea to create a non-unique index and optionally
-a foreign key constraint on the author column for the books table:
+If the associated object has already been retrieved from the database for this
+object, the cached version will be returned. To override this behavior (and
+force a database read), call `#reload_association` on the parent object.
```ruby
-create_table :books do |t|
- t.belongs_to :author, index: true, foreign_key: true
- # ...
-end
+@author = @book.reload_author
```
-### The `has_many :through` Association
-
-A [`has_many :through`][`has_many`] association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding _through_ a third model. For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this:
+To unload the cached version of the associated object—causing the next access,
+if any, to query it from the database—call `#reset_association` on the parent
+object.
```ruby
-class Physician < ApplicationRecord
- has_many :appointments
- has_many :patients, through: :appointments
-end
+@book.reset_author
+```
-class Appointment < ApplicationRecord
- belongs_to :physician
- belongs_to :patient
-end
+##### Assigning the Association
-class Patient < ApplicationRecord
- has_many :appointments
- has_many :physicians, through: :appointments
-end
+The `association=` method assigns an associated object to this object. Behind
+the scenes, this means extracting the primary key from the associated object and
+setting this object's foreign key to the same value.
+
+```ruby
+@book.author = @author
```
-
-The corresponding migration might look like this:
+The `build_association` method returns a new object of the associated type. This
+object will be instantiated from the passed attributes, and the link through
+this object's foreign key will be set, but the associated object will _not_ yet
+be saved.
```ruby
-class CreateAppointments < ActiveRecord::Migration[7.2]
- def change
- create_table :physicians do |t|
- t.string :name
- t.timestamps
- end
+@author = @book.build_author(author_number: 123,
+ author_name: "John Doe")
+```
- create_table :patients do |t|
- t.string :name
- t.timestamps
- end
+The `create_association` method takes it a step further and also saves the
+associated object once it passes all of the validations specified on the
+associated model.
- create_table :appointments do |t|
- t.belongs_to :physician
- t.belongs_to :patient
- t.datetime :appointment_date
- t.timestamps
- end
- end
-end
+```ruby
+@author = @book.create_author(author_number: 123,
+ author_name: "John Doe")
```
-The collection of join models can be managed via the [`has_many` association methods](#has-many-association-reference).
-For example, if you assign:
+Finally, `create_association!` does the same, but raises
+`ActiveRecord::RecordInvalid` if the record is invalid.
```ruby
-physician.patients = patients
+# This will raise ActiveRecord::RecordInvalid because the name is blank
+begin
+ @book.create_author!(author_number: 123, name: "")
+rescue ActiveRecord::RecordInvalid => e
+ puts e.message
+end
```
-Then new join models are automatically created for the newly associated objects.
-If some that existed previously are now missing, then their join rows are automatically deleted.
+```irb
+irb> raise_validation_error: Validation failed: Name can't be blank (ActiveRecord::RecordInvalid)
+```
-WARNING: Automatic deletion of join models is direct, no destroy callbacks are triggered.
+##### Checking for Association Changes
-The `has_many :through` association is also useful for setting up "shortcuts" through nested `has_many` associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way:
+The `association_changed?` method returns true if a new associated object has
+been assigned and the foreign key will be updated in the next save.
+
+The `association_previously_changed?` method returns true if the previous save
+updated the association to reference a new associate object.
```ruby
-class Document < ApplicationRecord
- has_many :sections
- has_many :paragraphs, through: :sections
-end
+@book.author # => #
+@book.author_changed? # => false
+@book.author_previously_changed? # => false
-class Section < ApplicationRecord
- belongs_to :document
- has_many :paragraphs
-end
+@book.author = Author.second # => #
+@book.author_changed? # => true
-class Paragraph < ApplicationRecord
- belongs_to :section
-end
+@book.save!
+@book.author_changed? # => false
+@book.author_previously_changed? # => true
```
-With `through: :sections` specified, Rails will now understand:
+NOTE: Do not confuse `model.association_changed?` with
+`model.association.changed?`. The former checks if the association has been
+replaced with a new record, while the latter tracks changes to the attributes of
+the association.
+
+##### Checking for Existing Associations
+
+You can see if any associated objects exist by using the `association.nil?`
+method:
```ruby
-@document.paragraphs
+if @book.author.nil?
+ @msg = "No author found for this book"
+end
```
-### The `has_one :through` Association
+##### Saving Behavior of Associated Objects
+
+Assigning an object to a `belongs_to` association does _not_ automatically save
+either the current object or the associated object. However, when you save the
+current object, the association is saved as well.
+
+### `has_one`
+
+A [`has_one`][] association indicates that one other model has a reference to
+this model. That model can be fetched through this association.
-A [`has_one :through`][`has_one`] association sets up a one-to-one connection with another model. This association indicates
-that the declaring model can be matched with one instance of another model by proceeding _through_ a third model.
-For example, if each supplier has one account, and each account is associated with one account history, then the
-supplier model could look like this:
+For example, if each supplier in your application has only one account, you'd
+declare the supplier model like this:
```ruby
class Supplier < ApplicationRecord
has_one :account
- has_one :account_history, through: :account
-end
-
-class Account < ApplicationRecord
- belongs_to :supplier
- has_one :account_history
-end
-
-class AccountHistory < ApplicationRecord
- belongs_to :account
end
```
-
+The main difference from `belongs_to` is that the link column (in this case
+`supplier_id`) is located in the other table, not the table where the `has_one`
+is declared.
+
+
The corresponding migration might look like this:
```ruby
-class CreateAccountHistories < ActiveRecord::Migration[7.2]
+class CreateSuppliers < ActiveRecord::Migration[7.2]
def change
create_table :suppliers do |t|
t.string :name
@@ -355,2288 +447,2333 @@ class CreateAccountHistories < ActiveRecord::Migration[7.2]
t.string :account_number
t.timestamps
end
-
- create_table :account_histories do |t|
- t.belongs_to :account
- t.integer :credit_rating
- t.timestamps
- end
end
end
```
-### The `has_and_belongs_to_many` Association
+The `has_one` association creates a one-to-one match with another model. In
+database terms, this association says that the other class contains the foreign
+key. If this class contains the foreign key, then you should use `belongs_to`
+instead.
-A [`has_and_belongs_to_many`][] association creates a direct many-to-many connection with another model, with no intervening model.
-This association indicates that each instance of the declaring model refers to zero or more instances of another model.
-For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:
+Depending on the use case, you might also need to create a unique index and/or a
+foreign key constraint on the supplier column for the accounts table. The unique
+index ensures that each supplier is associated with only one account and allows
+you to query in an efficient manner, while the foreign key constraint ensures
+that the `supplier_id` in the `accounts` table refers to a valid `supplier` in
+the `suppliers` table. This enforces the association at the database level.
```ruby
-class Assembly < ApplicationRecord
- has_and_belongs_to_many :parts
-end
-
-class Part < ApplicationRecord
- has_and_belongs_to_many :assemblies
+create_table :accounts do |t|
+ t.belongs_to :supplier, index: { unique: true }, foreign_key: true
+ # ...
end
```
-
-
-The corresponding migration might look like this:
-
-```ruby
-class CreateAssembliesAndParts < ActiveRecord::Migration[7.2]
- def change
- create_table :assemblies do |t|
- t.string :name
- t.timestamps
- end
+This relation can be [bi-directional](#bi-directional-associations) when used in
+combination with `belongs_to` on the other model.
- create_table :parts do |t|
- t.string :part_number
- t.timestamps
- end
+#### Methods Added by [`has_one`](#has-one)
- create_table :assemblies_parts, id: false do |t|
- t.belongs_to :assembly
- t.belongs_to :part
- end
- end
-end
-```
+When you declare a `has_one` association, the declaring class automatically
+gains numerous methods related to the association. Some of these are:
-### Choosing Between `belongs_to` and `has_one`
+* `association`
+* `association=(associate)`
+* `build_association(attributes = {})`
+* `create_association(attributes = {})`
+* `create_association!(attributes = {})`
+* `reload_association`
+* `reset_association`
-If you want to set up a one-to-one relationship between two models, you'll need to add `belongs_to` to one, and `has_one` to the other. How do you know which is which?
+We'll discuss some of the common methods, but you can find an exhaustive list in
+the [ActiveRecord Associations
+API](https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_one).
-The distinction is in where you place the foreign key (it goes on the table for the class declaring the `belongs_to` association), but you should give some thought to the actual meaning of the data as well. The `has_one` relationship says that one of something is yours - that is, that something points back to you. For example, it makes more sense to say that a supplier owns an account than that an account owns a supplier. This suggests that the correct relationships are like this:
+Like with the [`belongs_to` references](#methods-added-by-belongs-to), in all of
+these methods, `association` is replaced with the symbol passed as the first
+argument to `has_one`. For example, given the declaration:
```ruby
+# app/models/supplier.rb
class Supplier < ApplicationRecord
has_one :account
end
+# app/models/account.rb
class Account < ApplicationRecord
+ validates :terms, presence: true
belongs_to :supplier
end
```
-The corresponding migration might look like this:
-
-```ruby
-class CreateSuppliers < ActiveRecord::Migration[7.2]
- def change
- create_table :suppliers do |t|
- t.string :name
- t.timestamps
- end
+Each instance of the `Supplier` model will have these methods:
- create_table :accounts do |t|
- t.bigint :supplier_id
- t.string :account_number
- t.timestamps
- end
-
- add_index :accounts, :supplier_id
- end
-end
-```
+* `account`
+* `account=`
+* `build_account`
+* `create_account`
+* `create_account!`
+* `reload_account`
+* `reset_account`
-NOTE: Using `t.bigint :supplier_id` makes the foreign key naming obvious and explicit. In current versions of Rails, you can abstract away this implementation detail by using `t.references :supplier` instead.
+NOTE: When initializing a new `has_one` or `belongs_to` association you must use
+the `build_` prefix to build the association, rather than the
+`association.build` method that would be used for `has_many` or
+`has_and_belongs_to_many` associations. To create one, use the `create_` prefix.
-### Choosing Between `has_many :through` and `has_and_belongs_to_many`
+##### Retrieving the association
-Rails offers two different ways to declare a many-to-many relationship between models. The first way is to use `has_and_belongs_to_many`, which allows you to make the association directly:
+The `association` method returns the associated object, if any. If no associated
+object is found, it returns `nil`.
```ruby
-class Assembly < ApplicationRecord
- has_and_belongs_to_many :parts
-end
+@account = @supplier.account
+```
-class Part < ApplicationRecord
- has_and_belongs_to_many :assemblies
-end
+If the associated object has already been retrieved from the database for this
+object, the cached version will be returned. To override this behavior (and
+force a database read), call `#reload_association` on the parent object.
+
+```ruby
+@account = @supplier.reload_account
```
-The second way to declare a many-to-many relationship is to use `has_many :through`. This makes the association indirectly, through a join model:
+To unload the cached version of the associated object—forcing the next access,
+if any, to query it from the database—call `#reset_association` on the parent
+object.
```ruby
-class Assembly < ApplicationRecord
- has_many :manifests
- has_many :parts, through: :manifests
-end
+@supplier.reset_account
+```
-class Manifest < ApplicationRecord
- belongs_to :assembly
- belongs_to :part
-end
+##### Assigning the Association
-class Part < ApplicationRecord
- has_many :manifests
- has_many :assemblies, through: :manifests
-end
+The `association=` method assigns an associated object to this object. Behind
+the scenes, this means extracting the primary key from this object and setting
+the associated object's foreign key to the same value.
+
+```ruby
+@supplier.account = @account
```
-The simplest rule of thumb is that you should set up a `has_many :through` relationship if you need to work with the relationship model as an independent entity. If you don't need to do anything with the relationship model, it may be simpler to set up a `has_and_belongs_to_many` relationship (though you'll need to remember to create the joining table in the database).
-You should use `has_many :through` if you need validations, callbacks, or extra attributes on the join model.
+The `build_association` method returns a new object of the associated type. This
+object will be instantiated from the passed attributes, and the link through
+this objects foreign key will be set, but the associated object will _not_ yet
+be saved.
+
+```ruby
+@account = @supplier.build_account(terms: "Net 30")
+```
-While `has_and_belongs_to_many` suggests creating a join table with no primary key via `id: false`, consider using a composite primary key for the join table in the `has_many :through` relationship.
-For example, it's recommended to use `create_table :manifests, primary_key: [:assembly_id, :part_id]` in the example above.
+The `create_association` method takes it a step further and also saves the
+associated object once it passes all of the validations specified on the
+associated model.
-### Polymorphic Associations
+```ruby
+@account = @supplier.create_account(terms: "Net 30")
+```
-A slightly more advanced twist on associations is the _polymorphic association_. With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model. Here's how this could be declared:
+Finally, `create_association!` does the same, but raises
+`ActiveRecord::RecordInvalid` if the record is invalid.
+
+`create_association!` does the same as `create_association` above, but raises
+`ActiveRecord::RecordInvalid` if the record is invalid.
```ruby
-class Picture < ApplicationRecord
- belongs_to :imageable, polymorphic: true
+# This will raise ActiveRecord::RecordInvalid because the terms is blank
+begin
+ @supplier.create_account!(terms: "")
+rescue ActiveRecord::RecordInvalid => e
+ puts e.message
end
+```
-class Employee < ApplicationRecord
- has_many :pictures, as: :imageable
-end
+```irb
+irb> raise_validation_error: Validation failed: Terms can't be blank (ActiveRecord::RecordInvalid)
+```
-class Product < ApplicationRecord
- has_many :pictures, as: :imageable
+##### Checking for Existing Associations
+
+You can see if any associated objects exist by using the `association.nil?`
+method:
+
+```ruby
+if @supplier.account.nil?
+ @msg = "No account found for this supplier"
end
```
-You can think of a polymorphic `belongs_to` declaration as setting up an interface that any other model can use. From an instance of the `Employee` model, you can retrieve a collection of pictures: `@employee.pictures`.
+##### Saving Behavior of Associated Objects
-Similarly, you can retrieve `@product.pictures`.
+When you assign an object to a `has_one` association, that object is
+automatically saved to update its foreign key. Additionally, any object being
+replaced is also automatically saved, as its foreign key will change too.
-If you have an instance of the `Picture` model, you can get to its parent via `@picture.imageable`. To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface:
+If either of these saves fails due to validation errors, the assignment
+statement returns `false`, and the assignment itself is canceled.
-```ruby
-class CreatePictures < ActiveRecord::Migration[7.2]
- def change
- create_table :pictures do |t|
- t.string :name
- t.bigint :imageable_id
- t.string :imageable_type
- t.timestamps
- end
+If the parent object (the one declaring the `has_one` association) is unsaved
+(that is, `new_record?` returns `true`) then the child objects are not saved
+immediately. They will be automatically saved when the parent object is saved.
- add_index :pictures, [:imageable_type, :imageable_id]
- end
+If you want to assign an object to a `has_one` association without saving the
+object, use the `build_association` method. This method creates a new, unsaved
+instance of the associated object, allowing you to work with it before deciding
+to save it.
+
+Use `autosave: false` when you want to control the saving behavior of the
+associated objects for the model. This setting prevents the associated object
+from being saved automatically when the parent object is saved. In contrast, use
+`build_association` when you need to work with an unsaved associated object and
+delay its persistence until you're ready.
+
+### `has_many`
+
+A [`has_many`][] association is similar to `has_one`, but indicates a
+one-to-many relationship with another model. You'll often find this association
+on the "other side" of a `belongs_to` association. This association indicates
+that each instance of the model has zero or more instances of another model. For
+example, in an application containing authors and books, the author model could
+be declared like this:
+
+```ruby
+class Author < ApplicationRecord
+ has_many :books
end
```
-This migration can be simplified by using the `t.references` form:
+`has_many` establishes a one-to-many relationship between models, allowing each
+instance of the declaring model (`Author`) to have multiple instances of the
+associated model (`Book`).
+
+NOTE: Unlike a `has_one` and `belongs_to` association, the name of the other
+model is pluralized when declaring a `has_many` association.
+
+
+
+The corresponding migration might look like this:
```ruby
-class CreatePictures < ActiveRecord::Migration[7.2]
+class CreateAuthors < ActiveRecord::Migration[7.2]
def change
- create_table :pictures do |t|
+ create_table :authors do |t|
t.string :name
- t.references :imageable, polymorphic: true
+ t.timestamps
+ end
+
+ create_table :books do |t|
+ t.belongs_to :author
+ t.datetime :published_at
t.timestamps
end
end
end
```
-NOTE: Since polymorphic associations rely on storing class names in the
-database, that data must remain synchronized with the class name used by the
-Ruby code. When renaming a class, make sure to update the data in the
-polymorphic type column.
+The `has_many` association creates a one-to-many relationship with another
+model. In database terms, this association says that the other class will have a
+foreign key that refers to instances of this class.
-
+In this migration, the `authors` table is created with a `name` column to store
+the names of authors. The `books` table is also created, and it includes a
+`belongs_to :author` association. This association establishes a foreign key
+relationship between the `books` and `authors` tables. Specifically, the
+`author_id` column in the `books` table acts as a foreign key, referencing the
+`id` column in the `authors` table. By including this `belongs_to :author`
+association in the `books` table, we ensure that each book is associated with a
+single author, enabling a `has_many` association from the `Author` model. This
+setup allows each author to have multiple associated books.
-### Associations between Models with Composite Primary Keys
+Depending on the use case, it's usually a good idea to create a non-unique index
+and optionally a foreign key constraint on the author column for the books
+table. Adding an index on the `author_id` column improves query performance when
+retrieving books associated with a specific author.
-Rails is often able to infer the primary key - foreign key information between associated models with composite
-primary keys without needing extra information. Take the following example:
+If you wish to enforce [referential
+integrity](https://en.wikipedia.org/wiki/Referential_integrity) at the database
+level, add the [`foreign_key: true`](active_record_migrations.html#foreign-keys)
+option to the `reference` column declarations above. This will ensure that the
+`author_id` in the books table must correspond to a valid `id` in the `authors`
+table,
```ruby
-class Order < ApplicationRecord
- self.primary_key = [:shop_id, :id]
- has_many :books
-end
-
-class Book < ApplicationRecord
- belongs_to :order
+create_table :books do |t|
+ t.belongs_to :author, index: true, foreign_key: true
+ # ...
end
```
-Here, Rails assumes that the `:id` column should be used as the primary key for the association between an order
-and its books, just as with a regular `has_many` / `belongs_to` association. It will infer that the foreign key column
-on the `books` table is `:order_id`. Accessing a book's order:
+This relation can be [bi-directional](#bi-directional-associations) when used in
+combination with `belongs_to` on the other model.
-```ruby
-order = Order.create!(id: [1, 2], status: "pending")
-book = order.books.create!(title: "A Cool Book")
+#### Methods Added by `has_many`
-book.reload.order
-```
+When you declare a `has_many` association, the declaring class gains numerous
+methods related to the association. Some of these are:
-will generate the following SQL to access the order:
+* `collection`
+* [`collection<<(object, ...)`][`collection<<`]
+* [`collection.delete(object, ...)`][`collection.delete`]
+* [`collection.destroy(object, ...)`][`collection.destroy`]
+* `collection=(objects)`
+* `collection_singular_ids`
+* `collection_singular_ids=(ids)`
+* [`collection.clear`][]
+* [`collection.empty?`][]
+* [`collection.size`][]
+* [`collection.find(...)`][`collection.find`]
+* [`collection.where(...)`][`collection.where`]
+* [`collection.exists?(...)`][`collection.exists?`]
+* [`collection.build(attributes = {})`][`collection.build`]
+* [`collection.create(attributes = {})`][`collection.create`]
+* [`collection.create!(attributes = {})`][`collection.create!`]
+* [`collection.reload`][]
-```sql
-SELECT * FROM orders WHERE id = 2
-```
+We'll discuss some of the common methods, but you can find an exhaustive list in
+the [ActiveRecord Associations
+API](https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many).
-This only works if the model's composite primary key contains the `:id` column, _and_ the column is unique for
-all records. In order to use the full composite primary key in associations, set the `foreign_key:` option on
-the association. This option specifies a composite foreign key on the association: all columns in the foreign key will
-be used when querying the associated record(s). For example:
+In all of these methods, `collection` is replaced with the symbol passed as the
+first argument to `has_many`, and `collection_singular` is replaced with the
+singularized version of that symbol. For example, given the declaration:
```ruby
class Author < ApplicationRecord
- self.primary_key = [:first_name, :last_name]
- has_many :books, foreign_key: [:first_name, :last_name]
+ has_many :books
end
+```
-class Book < ApplicationRecord
- belongs_to :author, foreign_key: [:author_first_name, :author_last_name]
-end
+An instance of the `Author` model can have the following methods:
+
+```
+books
+books<<(object, ...)
+books.delete(object, ...)
+books.destroy(object, ...)
+books=(objects)
+book_ids
+book_ids=(ids)
+books.clear
+books.empty?
+books.size
+books.find(...)
+books.where(...)
+books.exists?(...)
+books.build(attributes = {}, ...)
+books.create(attributes = {})
+books.create!(attributes = {})
+books.reload
```
-Accessing a book's author:
+[`collection<<`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-3C-3C
+[`collection.build`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-build
+[`collection.clear`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-clear
+[`collection.create`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-create
+[`collection.create!`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-create-21
+[`collection.delete`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-delete
+[`collection.destroy`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-destroy
+[`collection.empty?`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-empty-3F
+[`collection.exists?`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/FinderMethods.html#method-i-exists-3F
+[`collection.find`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-find
+[`collection.reload`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-reload
+[`collection.size`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-size
+[`collection.where`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#method-i-where
+
+##### Managing the Collection
+
+The `collection` method returns a Relation of all of the associated objects. If
+there are no associated objects, it returns an empty Relation.
```ruby
-author = Author.create!(first_name: "Jane", last_name: "Doe")
-book = author.books.create!(title: "A Cool Book")
-
-book.reload.author
+@books = @author.books
```
-will use `:first_name` _and_ `:last_name` in the SQL query:
+The [`collection.delete`][] method removes one or more objects from the
+collection by setting their foreign keys to `NULL`.
-```sql
-SELECT * FROM authors WHERE first_name = 'Jane' AND last_name = 'Doe'
+```ruby
+@author.books.delete(@book1)
```
-### Self Joins
+WARNING: Additionally, objects will be destroyed if they're associated with
+`dependent: :destroy`, and deleted if they're associated with `dependent:
+:delete_all`.
-In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations:
+The [`collection.destroy`][] method removes one or more objects from the
+collection by running `destroy` on each object.
```ruby
-class Employee < ApplicationRecord
- has_many :subordinates, class_name: "Employee",
- foreign_key: "manager_id"
-
- belongs_to :manager, class_name: "Employee", optional: true
-end
+@author.books.destroy(@book1)
```
-With this setup, you can retrieve `@employee.subordinates` and `@employee.manager`.
+WARNING: Objects will _always_ be removed from the database, ignoring the
+`:dependent` option.
-In your migrations/schema, you will add a references column to the model itself.
+The [`collection.clear`][] method removes all objects from the collection
+according to the strategy specified by the `dependent` option. If no option is
+given, it follows the default strategy. The default strategy for `has_many
+:through` associations is `delete_all`, and for `has_many` associations is to
+set the foreign keys to `NULL`.
```ruby
-class CreateEmployees < ActiveRecord::Migration[7.2]
- def change
- create_table :employees do |t|
- t.references :manager, foreign_key: { to_table: :employees }
- t.timestamps
- end
- end
-end
+@author.books.clear
```
-NOTE: The `to_table` option passed to `foreign_key` and more are explained in [`SchemaStatements#add_reference`][connection.add_reference].
+WARNING: Objects will be deleted if they're associated with `dependent:
+:destroy` or `dependent: :destroy_async`, just like `dependent: :delete_all`.
-[connection.add_reference]: https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_reference
+The [`collection.reload`][] method returns a Relation of all of the associated
+objects, forcing a database read. If there are no associated objects, it returns
+an empty Relation.
-Tips, Tricks, and Warnings
---------------------------
+```ruby
+@books = @author.books.reload
+```
-Here are a few things you should know to make efficient use of Active Record associations in your Rails applications:
+##### Assigning the Collection
-* Controlling caching
-* Avoiding name collisions
-* Updating the schema
-* Controlling association scope
-* Bi-directional associations
+The `collection=(objects)` method makes the collection contain only the supplied
+objects, by adding and deleting as appropriate. The changes are persisted to the
+database.
-### Controlling Caching
+The `collection_singular_ids=(ids)` method makes the collection contain only the
+objects identified by the supplied primary key values, by adding and deleting as
+appropriate. The changes are persisted to the database.
-All of the association methods are built around caching, which keeps the result of the most recent query available for further operations. The cache is even shared across methods. For example:
+##### Querying the Collection
+
+The `collection_singular_ids` method returns an array of the ids of the objects
+in the collection.
```ruby
-# retrieves books from the database
-author.books.load
+@book_ids = @author.book_ids
+```
-# uses the cached copy of books
-author.books.size
+The [`collection.empty?`][] method returns `true` if the collection does not
+contain any associated objects.
-# uses the cached copy of books
-author.books.empty?
+```erb
+<% if @author.books.empty? %>
+ No Books Found
+<% end %>
```
-But what if you want to reload the cache, because data might have been changed by some other part of the application? Just call `reload` on the association:
+The [`collection.size`][] method returns the number of objects in the
+collection.
```ruby
-# retrieves books from the database
-author.books.load
+@book_count = @author.books.size
+```
-# uses the cached copy of books
-author.books.size
+The [`collection.find`][] method finds objects within the collection's table.
-# discards the cached copy of books and goes back to the database
-author.books.reload.empty?
+```ruby
+@available_book = @author.books.find(1)
```
-### Avoiding Name Collisions
-
-You are not free to use just any name for your associations. Because creating an association adds a method with that name to the model, it is a bad idea to give an association a name that is already used for an instance method of `ActiveRecord::Base`. The association method would override the base method and break things. For instance, `attributes` or `connection` are bad names for associations.
+The [`collection.where`][] method finds objects within the collection based on
+the conditions supplied but the objects are loaded lazily meaning that the
+database is queried only when the object(s) are accessed.
-### Updating the Schema
+```ruby
+@available_books = @author.books.where(available: true) # No query yet
+@available_book = @available_books.first # Now the database will be queried
+```
-Associations are extremely useful, but they are not magic. You are responsible for maintaining your database schema to match your associations. In practice, this means two things, depending on what sort of associations you are creating. For `belongs_to` associations you need to create foreign keys, and for `has_and_belongs_to_many` associations you need to create the appropriate join table.
+The [`collection.exists?`][] method checks whether an object meeting the
+supplied conditions exists in the collection's table.
-#### Creating Foreign Keys for `belongs_to` Associations
+##### Building and Creating Associated Objects
-When you declare a `belongs_to` association, you need to create foreign keys as appropriate. For example, consider this model:
+The [`collection.build`][] method returns a single or array of new objects of
+the associated type. The object(s) will be instantiated from the passed
+attributes, and the link through their foreign key will be created, but the
+associated objects will _not_ yet be saved.
```ruby
-class Book < ApplicationRecord
- belongs_to :author
-end
+@book = @author.books.build(published_at: Time.now,
+ book_number: "A12345")
+
+@books = @author.books.build([
+ { published_at: Time.now, book_number: "A12346" },
+ { published_at: Time.now, book_number: "A12347" }
+])
```
-This declaration needs to be backed up by a corresponding foreign key column in the books table. For a brand new table, the migration might look something like this:
+The [`collection.create`][] method returns a single or array of new objects of
+the associated type. The object(s) will be instantiated from the passed
+attributes, the link through its foreign key will be created, and, once it
+passes all of the validations specified on the associated model, the associated
+object _will_ be saved.
```ruby
-class CreateBooks < ActiveRecord::Migration[7.2]
- def change
- create_table :books do |t|
- t.datetime :published_at
- t.string :book_number
- t.references :author
- end
- end
-end
+@book = @author.books.create(published_at: Time.now,
+ book_number: "A12345")
+
+@books = @author.books.create([
+ { published_at: Time.now, book_number: "A12346" },
+ { published_at: Time.now, book_number: "A12347" }
+])
```
-Whereas for an existing table, it might look like this:
+`collection.create!` does the same as `collection.create`, but raises
+`ActiveRecord::RecordInvalid` if the record is invalid.
-```ruby
-class AddAuthorToBooks < ActiveRecord::Migration[7.2]
- def change
- add_reference :books, :author
- end
-end
-```
+##### When are Objects Saved?
-NOTE: If you wish to [enforce referential integrity at the database level][foreign_keys], add the `foreign_key: true` option to the ‘reference’ column declarations above.
+When you assign an object to a `has_many` association, that object is
+automatically saved (in order to update its foreign key). If you assign multiple
+objects in one statement, then they are all saved.
-[foreign_keys]: active_record_migrations.html#foreign-keys
+If any of these saves fails due to validation errors, then the assignment
+statement returns `false` and the assignment itself is cancelled.
-#### Creating Join Tables for `has_and_belongs_to_many` Associations
+If the parent object (the one declaring the `has_many` association) is unsaved
+(that is, `new_record?` returns `true`) then the child objects are not saved
+when they are added. All unsaved members of the association will automatically
+be saved when the parent is saved.
-If you create a `has_and_belongs_to_many` association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the `:join_table` option, Active Record creates the name by using the lexical order of the class names. So a join between author and book models will give the default join table name of "authors_books" because "a" outranks "b" in lexical ordering.
+If you want to assign an object to a `has_many` association without saving the
+object, use the `collection.build` method.
-WARNING: The precedence between model names is calculated using the `<=>` operator for `String`. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '\_' is lexicographically _less_ than 's' in common encodings).
+### `has_many :through`
-Whatever the name, you must manually generate the join table with an appropriate migration. For example, consider these associations:
+A [`has_many :through`][`has_many`] association is often used to set up a
+many-to-many relationship with another model. This association indicates that
+the declaring model can be matched with zero or more instances of another model
+by proceeding _through_ a third model.
-```ruby
-class Assembly < ApplicationRecord
- has_and_belongs_to_many :parts
-end
-
-class Part < ApplicationRecord
- has_and_belongs_to_many :assemblies
-end
-```
-
-These need to be backed up by a migration to create the `assemblies_parts` table. This table should be created without a primary key:
+For example, consider a medical practice where patients make appointments to see
+physicians. The relevant association declarations could look like this:
```ruby
-class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[7.2]
- def change
- create_table :assemblies_parts, id: false do |t|
- t.bigint :assembly_id
- t.bigint :part_id
- end
-
- add_index :assemblies_parts, :assembly_id
- add_index :assemblies_parts, :part_id
- end
+class Physician < ApplicationRecord
+ has_many :appointments
+ has_many :patients, through: :appointments
end
-```
-
-We pass `id: false` to `create_table` because that table does not represent a model. That's required for the association to work properly. If you observe any strange behavior in a `has_and_belongs_to_many` association like mangled model IDs, or exceptions about conflicting IDs, chances are you forgot that bit.
-For simplicity, you can also use the method `create_join_table`:
-
-```ruby
-class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[7.2]
- def change
- create_join_table :assemblies, :parts do |t|
- t.index :assembly_id
- t.index :part_id
- end
- end
+class Appointment < ApplicationRecord
+ belongs_to :physician
+ belongs_to :patient
end
-```
-
-### Controlling Association Scope
-
-By default, associations look for objects only within the current module's scope. This can be important when you declare Active Record models within a module. For example:
-
-```ruby
-module MyApplication
- module Business
- class Supplier < ApplicationRecord
- has_one :account
- end
- class Account < ApplicationRecord
- belongs_to :supplier
- end
- end
+class Patient < ApplicationRecord
+ has_many :appointments
+ has_many :physicians, through: :appointments
end
```
-This will work fine, because both the `Supplier` and the `Account` class are defined within the same scope (`MyApplication::Business`). This organization allows structuring models into folders based on their scope, without having to explicitly add the scope to every association:
+`has_many :through` establishes a many-to-many relationship between models,
+allowing instances of one model (Physician) to be associated with multiple
+instances of another model (Patient) through a third "join" model (Appointment).
-```ruby
-# app/models/my_application/business/supplier.rb
-module MyApplication
- module Business
- class Supplier < ApplicationRecord
- has_one :account
- end
- end
-end
-```
+
-```ruby
-# app/models/my_application/business/account.rb
-module MyApplication
- module Business
- class Account < ApplicationRecord
- belongs_to :supplier
- end
- end
-end
-```
-
-It is crucial to note that this does not affect the naming of your tables. For instance, if there is a `MyApplication::Business::Supplier` model, there must also be a `my_application_business_suppliers` table.
-
-Note that the following will _not_ work, because `Supplier` and `Account` are defined in different scopes (`MyApplication::Business` and `MyApplication::Billing`):
+The corresponding migration might look like this:
```ruby
-module MyApplication
- module Business
- class Supplier < ApplicationRecord
- has_one :account
- end
- end
-
- module Billing
- class Account < ApplicationRecord
- belongs_to :supplier
+class CreateAppointments < ActiveRecord::Migration[7.2]
+ def change
+ create_table :physicians do |t|
+ t.string :name
+ t.timestamps
end
- end
-end
-```
-To associate a model with a model in a different namespace, you must specify the complete class name in your association declaration:
-
-```ruby
-module MyApplication
- module Business
- class Supplier < ApplicationRecord
- has_one :account,
- class_name: "MyApplication::Billing::Account"
+ create_table :patients do |t|
+ t.string :name
+ t.timestamps
end
- end
- module Billing
- class Account < ApplicationRecord
- belongs_to :supplier,
- class_name: "MyApplication::Business::Supplier"
+ create_table :appointments do |t|
+ t.belongs_to :physician
+ t.belongs_to :patient
+ t.datetime :appointment_date
+ t.timestamps
end
end
end
```
-### Bi-directional Associations
-
-It's normal for associations to work in two directions, requiring declaration on two different models:
-
-```ruby
-class Author < ApplicationRecord
- has_many :books
-end
-
-class Book < ApplicationRecord
- belongs_to :author
-end
-```
-
-Active Record will attempt to automatically identify that these two models share
-a bi-directional association based on the association name. This information
-allows Active Record to:
-
-* Prevent needless queries for already-loaded data:
-
- ```irb
- irb> author = Author.first
- irb> author.books.all? do |book|
- irb> book.author.equal?(author) # No additional queries executed here
- irb> end
- => true
- ```
-
-* Prevent inconsistent data (since there is only one copy of the `Author` object
- loaded):
-
- ```irb
- irb> author = Author.first
- irb> book = author.books.first
- irb> author.name == book.author.name
- => true
- irb> author.name = "Changed Name"
- irb> author.name == book.author.name
- => true
- ```
-
-* Autosave associations in more cases:
-
- ```irb
- irb> author = Author.new
- irb> book = author.books.new
- irb> book.save!
- irb> book.persisted?
- => true
- irb> author.persisted?
- => true
- ```
-
-* Validate the [presence](active_record_validations.html#presence) and
- [absence](active_record_validations.html#absence) of associations in more
- cases:
-
- ```irb
- irb> book = Book.new
- irb> book.valid?
- => false
- irb> book.errors.full_messages
- => ["Author must exist"]
- irb> author = Author.new
- irb> book = author.books.new
- irb> book.valid?
- => true
- ```
-
-Active Record supports automatic identification for most associations with standard names. However, bi-directional associations that contain the `:through` or `:foreign_key` options will not be automatically identified.
-
-Custom scopes on the opposite association also prevent automatic identification, as do custom scopes on the association itself unless [`config.active_record.automatic_scope_inversing`][] is set to true (the default for new applications).
-
-For example, consider the following model declarations:
-
-```ruby
-class Author < ApplicationRecord
- has_many :books
-end
-
-class Book < ApplicationRecord
- belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
-end
-```
-
-Because of the `:foreign_key` option, Active Record will no longer automatically
-recognize the bi-directional association. This can cause your application to:
+In this migration the `physicians` and `patients` tables are created with a
+`name` column. The `appointments` table, which acts as the join table, is
+created with `physician_id` and `patient_id` columns, establishing the
+many-to-many relationship between `physicians` and `patients`.
-* Execute needless queries for the same data (in this example causing N+1 queries):
-
- ```irb
- irb> author = Author.first
- irb> author.books.any? do |book|
- irb> book.writer.equal?(author) # This executes an author query for every book
- irb> end
- => false
- ```
-
-* Reference multiple copies of a model with inconsistent data:
-
- ```irb
- irb> author = Author.first
- irb> book = author.books.first
- irb> author.name == book.writer.name
- => true
- irb> author.name = "Changed Name"
- irb> author.name == book.writer.name
- => false
- ```
-
-* Fail to autosave associations:
-
- ```irb
- irb> author = Author.new
- irb> book = author.books.new
- irb> book.save!
- irb> book.persisted?
- => true
- irb> author.persisted?
- => false
- ```
-
-* Fail to validate presence or absence:
-
- ```irb
- irb> author = Author.new
- irb> book = author.books.new
- irb> book.valid?
- => false
- irb> book.errors.full_messages
- => ["Author must exist"]
- ```
-
-Active Record provides the `:inverse_of` option so you can explicitly declare bi-directional associations:
+You could also consider using a [composite primary
+key](active_record_composite_primary_keys.html) for the join table in the
+`has_many :through` relationship like below:
```ruby
-class Author < ApplicationRecord
- has_many :books, inverse_of: 'writer'
-end
-
-class Book < ApplicationRecord
- belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
-end
-```
-
-By including the `:inverse_of` option in the `has_many` association declaration,
-Active Record will now recognize the bi-directional association and behave as in
-the initial examples above.
-
-[`config.active_record.automatic_scope_inversing`]: configuring.html#config-active-record-automatic-scope-inversing
-
-Detailed Association Reference
-------------------------------
-
-The following sections give the details of each type of association, including the methods that they add and the options that you can use when declaring an association.
-
-### `belongs_to` Association Reference
-
-In database terms, the `belongs_to` association says that this model's table contains a column which represents a reference to another table.
-This can be used to set up one-to-one or one-to-many relations, depending on the setup.
-If the table of the other class contains the reference in a one-to-one relation, then you should use `has_one` instead.
-
-#### Methods Added by `belongs_to`
-
-When you declare a `belongs_to` association, the declaring class automatically gains 8 methods related to the association:
-
-* `association`
-* `association=(associate)`
-* `build_association(attributes = {})`
-* `create_association(attributes = {})`
-* `create_association!(attributes = {})`
-* `reload_association`
-* `reset_association`
-* `association_changed?`
-* `association_previously_changed?`
-
-In all of these methods, `association` is replaced with the symbol passed as the first argument to `belongs_to`. For example, given the declaration:
-
-```ruby
-class Book < ApplicationRecord
- belongs_to :author
+class CreateAppointments < ActiveRecord::Migration[7.2]
+ def change
+ # ...
+ create_table :appointments, primary_key: [:physician_id, :patient_id] do |t|
+ t.belongs_to :physician
+ t.belongs_to :patient
+ t.datetime :appointment_date
+ t.timestamps
+ end
+ end
end
```
-Each instance of the `Book` model will have these methods:
-
-* `author`
-* `author=`
-* `build_author`
-* `create_author`
-* `create_author!`
-* `reload_author`
-* `reset_author`
-* `author_changed?`
-* `author_previously_changed?`
-
-NOTE: When initializing a new `has_one` or `belongs_to` association you must use the `build_` prefix to build the association, rather than the `association.build` method that would be used for `has_many` or `has_and_belongs_to_many` associations. To create one, use the `create_` prefix.
-
-##### `association`
-
-The `association` method returns the associated object, if any. If no associated object is found, it returns `nil`.
-
-```ruby
-@author = @book.author
-```
-
-If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), call `#reload_association` on the parent object.
-
-```ruby
-@author = @book.reload_author
-```
-
-To unload the cached version of the associated object—causing the next access, if any, to query it from the database—call `#reset_association` on the parent object.
-
-```ruby
-@book.reset_author
-```
-
-##### `association=(associate)`
-
-The `association=` method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from the associated object and setting this object's foreign key to the same value.
-
-```ruby
-@book.author = @author
-```
-
-##### `build_association(attributes = {})`
-
-The `build_association` method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through this object's foreign key will be set, but the associated object will _not_ yet be saved.
-
-```ruby
-@author = @book.build_author(author_number: 123,
- author_name: "John Doe")
-```
-
-##### `create_association(attributes = {})`
-
-The `create_association` method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through this object's foreign key will be set, and, once it passes all of the validations specified on the associated model, the associated object _will_ be saved.
+The collection of join models in a `has_many :through` association can be
+managed using standard [`has_many` association
+methods](#methods-added-by-has-many). For example, if you assign a list of
+patients to a physician like this:
```ruby
-@author = @book.create_author(author_number: 123,
- author_name: "John Doe")
-```
-
-##### `create_association!(attributes = {})`
-
-Does the same as `create_association` above, but raises `ActiveRecord::RecordInvalid` if the record is invalid.
-
-##### `association_changed?`
-
-The `association_changed?` method returns true if a new associated object has been assigned and the foreign key will be updated in the next save.
-
-```ruby
-@book.author # => #
-@book.author_changed? # => false
-
-@book.author = Author.second # => #
-@book.author_changed? # => true
-
-@book.save!
-@book.author_changed? # => false
-```
-
-##### `association_previously_changed?`
-
-The `association_previously_changed?` method returns true if the previous save updated the association to reference a new associate object.
-
-```ruby
-@book.author # => #
-@book.author_previously_changed? # => false
-
-@book.author = Author.second # => #
-@book.save!
-@book.author_previously_changed? # => true
-```
-
-#### Options for `belongs_to`
-
-While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `belongs_to` association reference. Such customizations can easily be accomplished by passing options and scope blocks when you create the association. For example, this association uses two such options:
-
-```ruby
-class Book < ApplicationRecord
- belongs_to :author, touch: :books_updated_at,
- counter_cache: true
-end
+physician.patients = patients
```
-The [`belongs_to`][] association supports these options:
-
-* `:autosave`
-* `:class_name`
-* `:counter_cache`
-* `:default`
-* `:dependent`
-* `:ensuring_owner_was`
-* `:foreign_key`
-* `:foreign_type`
-* `:primary_key`
-* `:inverse_of`
-* `:optional`
-* `:polymorphic`
-* `:required`
-* `:strict_loading`
-* `:touch`
-* `:validate`
-
-##### `:autosave`
+Rails will automatically create new join models for any patients in the new list
+that were not previously associated with the physician. Additionally, if any
+patients that were previously associated with the physician are not included in
+the new list, their join records will be automatically deleted. This simplifies
+managing many-to-many relationships by handling the creation and deletion of the
+join models for you.
-If you set the `:autosave` option to `true`, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting `:autosave` to `false` is not the same as not setting the `:autosave` option. If the `:autosave` option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
+WARNING: Automatic deletion of join models is direct, no destroy callbacks are
+triggered. You can read more about callbacks in the [Active Record Callbacks
+Guide](active_record_callbacks.html).
-##### `:class_name`
+The `has_many :through` association is also useful for setting up "shortcuts"
+through nested `has_many` associations. This is particularly beneficial when you
+need to access a collection of related records through an intermediary
+association.
-If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if a book belongs to an author, but the actual name of the model containing authors is `Patron`, you'd set things up this way:
+For example, if a document has many sections, and each section has many
+paragraphs, you may sometimes want to get a simple collection of all paragraphs
+in the document without having to manually traverse through each section.
-```ruby
-class Book < ApplicationRecord
- belongs_to :author, class_name: "Patron"
-end
-```
-
-##### `:counter_cache`
-
-The `:counter_cache` option can be used to make finding the number of belonging objects more efficient. Consider these models:
+You can set this up with a `has_many :through` association as follows:
```ruby
-class Book < ApplicationRecord
- belongs_to :author
+class Document < ApplicationRecord
+ has_many :sections
+ has_many :paragraphs, through: :sections
end
-class Author < ApplicationRecord
- has_many :books
-end
-```
-
-With these declarations, asking for the value of `@author.books.size` requires making a call to the database to perform a `COUNT(*)` query. To avoid this call, you can add a counter cache to the _belonging_ model:
-
-```ruby
-class Book < ApplicationRecord
- belongs_to :author, counter_cache: true
+class Section < ApplicationRecord
+ belongs_to :document
+ has_many :paragraphs
end
-class Author < ApplicationRecord
- has_many :books
+class Paragraph < ApplicationRecord
+ belongs_to :section
end
```
-With this declaration, Rails will keep the cache value up to date, and then return that value in response to the `size` method.
-
-Although the `:counter_cache` option is specified on the model that includes
-the `belongs_to` declaration, the actual column must be added to the
-_associated_ (`has_many`) model. In the case above, you would need to add a
-column named `books_count` to the `Author` model.
-
-You can override the default column name by specifying a custom column name in
-the `counter_cache` declaration instead of `true`. For example, to use
-`count_of_books` instead of `books_count`:
+With `through: :sections` specified, Rails will now understand:
```ruby
-class Book < ApplicationRecord
- belongs_to :author, counter_cache: :count_of_books
-end
-
-class Author < ApplicationRecord
- has_many :books
-end
+@document.paragraphs
```
-NOTE: You only need to specify the `:counter_cache` option on the `belongs_to`
-side of the association.
-
-Starting to use counter caches on existing large tables can be troublesome, because the column
-values must be backfilled separately of the column addition (to not lock the table for too long)
-and before the use of `:counter_cache` (otherwise methods like `size`/`any?`/etc, which use
-counter caches internally, can produce incorrect results). To safely backfill the values while
-keeping counter cache columns updated with the child records creation/removal and to avoid the
-mentioned methods use the possibly incorrect counter cache column values and always get the results
-from the database, use `counter_cache: { active: false }`. If you also need to specify a custom
-column name, use `counter_cache: { active: false, column: :my_custom_counter }`.
-
-If for some reason you change the value of an owner model's primary key, and do
-not also update the foreign keys of the counted models, then the counter cache
-may have stale data. In other words, any orphaned models will still count
-towards the counter. To fix a stale counter cache, use [`reset_counters`][].
-
-[`reset_counters`]: https://api.rubyonrails.org/classes/ActiveRecord/CounterCache/ClassMethods.html#method-i-reset_counters
-
-##### `:default`
-
-When set to `true`, the association will not have its presence validated.
-
-##### `:dependent`
-
-If you set the `:dependent` option to:
-
-* `:destroy`, when the object is destroyed, `destroy` will be called on its
- associated objects.
-* `:delete`, when the object is destroyed, all its associated objects will be
- deleted directly from the database without calling their `destroy` method.
-* `:destroy_async`: when the object is destroyed, an `ActiveRecord::DestroyAssociationAsyncJob`
- job is enqueued which will call destroy on its associated objects. Active Job must be set up
- for this to work. Do not use this option if the association is backed by foreign key
- constraints in your database. The foreign key constraint actions will occur inside the same
- transaction that deletes its owner.
-
-WARNING: You should not specify this option on a `belongs_to` association that is connected with a `has_many` association on the other class. Doing so can lead to orphaned records in your database.
-
-##### `:ensuring_owner_was`
-
-Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.
-
-##### `:foreign_key`
-
-By convention, Rails assumes that the column used to hold the foreign key on this model is the name of the association with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly:
+Whereas, if you had not set up a `has_many :through` association, you would have
+needed to do something like this to get paragraphs in a document:
```ruby
-class Book < ApplicationRecord
- belongs_to :author, class_name: "Patron",
- foreign_key: "patron_id"
+paragraphs = []
+@document.sections.each do |section|
+ paragraphs.concat(section.paragraphs)
end
```
-TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
-
-##### `:foreign_type`
-
-Specify the column used to store the associated object’s type, if this is a polymorphic association. By default this is guessed to be the name of the association with a “`_type`” suffix. So a class that defines a `belongs_to :taggable, polymorphic: true` association will use “`taggable_type`” as the default `:foreign_type`.
+### `has_one :through`
-##### `:primary_key`
+A [`has_one :through`][`has_one`] association sets up a one-to-one relationship
+with another model through an intermediary model. This association indicates
+that the declaring model can be matched with one instance of another model by
+proceeding _through_ a third model.
-By convention, Rails assumes that the `id` column is used to hold the primary key
-of its tables. The `:primary_key` option allows you to specify a different column.
-
-For example, given we have a `users` table with `guid` as the primary key. If we want a separate `todos` table to hold the foreign key `user_id` in the `guid` column, then we can use `primary_key` to achieve this like so:
+For example, if each supplier has one account, and each account is associated
+with one account history, then the supplier model could look like this:
```ruby
-class User < ApplicationRecord
- self.primary_key = 'guid' # primary key is guid and not id
+class Supplier < ApplicationRecord
+ has_one :account
+ has_one :account_history, through: :account
end
-class Todo < ApplicationRecord
- belongs_to :user, primary_key: 'guid'
-end
-```
-
-When we execute `@user.todos.create` then the `@todo` record will have its
-`user_id` value as the `guid` value of `@user`.
-
-##### `:inverse_of`
-
-The `:inverse_of` option specifies the name of the `has_many` or `has_one` association that is the inverse of this association.
-See the [bi-directional association](#bi-directional-associations) section for more details.
-
-```ruby
-class Author < ApplicationRecord
- has_many :books, inverse_of: :author
+class Account < ApplicationRecord
+ belongs_to :supplier
+ has_one :account_history
end
-class Book < ApplicationRecord
- belongs_to :author, inverse_of: :books
+class AccountHistory < ApplicationRecord
+ belongs_to :account
end
```
-##### `:optional`
-
-If you set the `:optional` option to `true`, then the presence of the associated
-object won't be validated. By default, this option is set to `false`.
-
-##### `:polymorphic`
-
-Passing `true` to the `:polymorphic` option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail earlier in this guide.
+This setup allows a `supplier` to directly access its `account_history` through
+its `account`.
+
-##### `:required`
-
-When set to `true`, the association will also have its presence validated. This will validate the association itself, not the id. You can use `:inverse_of` to avoid an extra query during validation.
-
-NOTE: required is set to `true` by default and is deprecated. If you don’t want to have association presence validated, use `optional: true`.
-
-##### `:strict_loading`
-
-Enforces strict loading every time the associated record is loaded through this association.
-
-##### `:touch`
-
-If you set the `:touch` option to `true`, then the `updated_at` or `updated_on` timestamp on the associated object will be set to the current time whenever this object is saved or destroyed:
-
-```ruby
-class Book < ApplicationRecord
- belongs_to :author, touch: true
-end
-
-class Author < ApplicationRecord
- has_many :books
-end
-```
-
-In this case, saving or destroying a book will update the timestamp on the associated author. You can also specify a particular timestamp attribute to update:
+The corresponding migration to set up these associations might look like this:
```ruby
-class Book < ApplicationRecord
- belongs_to :author, touch: :books_updated_at
-end
-```
-
-##### `:validate`
-
-If you set the `:validate` option to `true`, then new associated objects will be validated whenever you save this object. By default, this is `false`: new associated objects will not be validated when this object is saved.
-
-#### Scopes for `belongs_to`
+class CreateAccountHistories < ActiveRecord::Migration[7.2]
+ def change
+ create_table :suppliers do |t|
+ t.string :name
+ t.timestamps
+ end
-There may be times when you wish to customize the query used by `belongs_to`. Such customizations can be achieved via a scope block. For example:
+ create_table :accounts do |t|
+ t.belongs_to :supplier
+ t.string :account_number
+ t.timestamps
+ end
-```ruby
-class Book < ApplicationRecord
- belongs_to :author, -> { where active: true }
+ create_table :account_histories do |t|
+ t.belongs_to :account
+ t.integer :credit_rating
+ t.timestamps
+ end
+ end
end
```
-You can use any of the standard [querying methods](active_record_querying.html) inside the scope block. The following ones are discussed below:
+### `has_and_belongs_to_many`
-* `where`
-* `includes`
-* `readonly`
-* `select`
+A [`has_and_belongs_to_many`][] association creates a direct many-to-many
+relationship with another model, with no intervening model. This association
+indicates that each instance of the declaring model refers to zero or more
+instances of another model.
-##### `where`
-
-The `where` method lets you specify the conditions that the associated object must meet.
+For example, consider an application with `Assembly` and `Part` models, where
+each assembly can contain many parts, and each part can be used in many
+assemblies. You can set up the models as follows:
```ruby
-class Book < ApplicationRecord
- belongs_to :author, -> { where active: true }
-end
-```
-
-##### `includes`
-
-You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
-
-```ruby
-class Chapter < ApplicationRecord
- belongs_to :book
-end
-
-class Book < ApplicationRecord
- belongs_to :author
- has_many :chapters
+class Assembly < ApplicationRecord
+ has_and_belongs_to_many :parts
end
-class Author < ApplicationRecord
- has_many :books
+class Part < ApplicationRecord
+ has_and_belongs_to_many :assemblies
end
```
-If you frequently retrieve authors directly from chapters (`@chapter.book.author`), then you can make your code somewhat more efficient by including authors in the association from chapters to books:
+
+
+Even though a `has_and_belongs_to_many` does not require an intervening model,
+it does require a separate table to establish the many-to-many relationship
+between the two models involved. This intervening table serves to store the
+related data, mapping the associations between instances of the two models. The
+table does not necessarily need a primary key since its purpose is solely to
+manage the relationship between the associated records. The corresponding
+migration might look like this:
```ruby
-class Chapter < ApplicationRecord
- belongs_to :book, -> { includes :author }
-end
+class CreateAssembliesAndParts < ActiveRecord::Migration[7.2]
+ def change
+ create_table :assemblies do |t|
+ t.string :name
+ t.timestamps
+ end
-class Book < ApplicationRecord
- belongs_to :author
- has_many :chapters
-end
+ create_table :parts do |t|
+ t.string :part_number
+ t.timestamps
+ end
-class Author < ApplicationRecord
- has_many :books
+ # Create a join table to establish the many-to-many relationship between assemblies and parts.
+ # `id: false` indicates that the table does not need a primary key of its own
+ create_table :assemblies_parts, id: false do |t|
+ # creates foreign keys linking the join table to the `assemblies` and `parts` tables
+ t.belongs_to :assembly
+ t.belongs_to :part
+ end
+ end
end
```
-NOTE: There's no need to use `includes` for immediate associations - that is, if you have `Book belongs_to :author`, then the author is eager-loaded automatically when it's needed.
+The `has_and_belongs_to_many` association creates a many-to-many relationship
+with another model. In database terms, this associates two classes via an
+intermediate join table that includes foreign keys referring to each of the
+classes.
-##### `readonly`
+If the join table for a `has_and_belongs_to_many` association has additional
+columns beyond the two foreign keys, these columns will be added as attributes
+to records retrieved via that association. Records returned with additional
+attributes will always be read-only, because Rails cannot save changes to those
+attributes.
-If you use `readonly`, then the associated object will be read-only when retrieved via the association.
+WARNING: The use of extra attributes on the join table in a
+`has_and_belongs_to_many` association is deprecated. If you require this sort of
+complex behavior on the table that joins two models in a many-to-many
+relationship, you should use a `has_many :through` association instead of
+`has_and_belongs_to_many`.
-##### `select`
+#### Methods Added by `has_and_belongs_to_many`
-The `select` method lets you override the SQL `SELECT` clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.
+When you declare a `has_and_belongs_to_many` association, the declaring class
+gains numerous methods related to the association. Some of these are:
-TIP: If you use the `select` method on a `belongs_to` association, you should also set the `:foreign_key` option to guarantee the correct results.
+* `collection`
+* [`collection<<(object, ...)`][`collection<<`]
+* [`collection.delete(object, ...)`][`collection.delete`]
+* [`collection.destroy(object, ...)`][`collection.destroy`]
+* `collection=(objects)`
+* `collection_singular_ids`
+* `collection_singular_ids=(ids)`
+* [`collection.clear`][]
+* [`collection.empty?`][]
+* [`collection.size`][]
+* [`collection.find(...)`][`collection.find`]
+* [`collection.where(...)`][`collection.where`]
+* [`collection.exists?(...)`][`collection.exists?`]
+* [`collection.build(attributes = {})`][`collection.build`]
+* [`collection.create(attributes = {})`][`collection.create`]
+* [`collection.create!(attributes = {})`][`collection.create!`]
+* [`collection.reload`][]
-#### Do Any Associated Objects Exist?
+We'll discuss some of the common methods, but you can find an exhaustive list in
+the [ActiveRecord Associations
+API](https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many).
-You can see if any associated objects exist by using the `association.nil?` method:
+In all of these methods, `collection` is replaced with the symbol passed as the
+first argument to `has_and_belongs_to_many`, and `collection_singular` is
+replaced with the singularized version of that symbol. For example, given the
+declaration:
```ruby
-if @book.author.nil?
- @msg = "No author found for this book"
+class Part < ApplicationRecord
+ has_and_belongs_to_many :assemblies
end
```
-#### When are Objects Saved?
-
-Assigning an object to a `belongs_to` association does _not_ automatically save the object. It does not save the associated object either.
-
-### `has_one` Association Reference
+An instance of the `Part` model can have the following methods:
-The `has_one` association creates a one-to-one match with another model. In database terms, this association says that the other class contains the foreign key. If this class contains the foreign key, then you should use `belongs_to` instead.
-
-#### Methods Added by `has_one`
-
-When you declare a `has_one` association, the declaring class automatically gains 6 methods related to the association:
+```
+assemblies
+assemblies<<(object, ...)
+assemblies.delete(object, ...)
+assemblies.destroy(object, ...)
+assemblies=(objects)
+assembly_ids
+assembly_ids=(ids)
+assemblies.clear
+assemblies.empty?
+assemblies.size
+assemblies.find(...)
+assemblies.where(...)
+assemblies.exists?(...)
+assemblies.build(attributes = {}, ...)
+assemblies.create(attributes = {})
+assemblies.create!(attributes = {})
+assemblies.reload
+```
-* `association`
-* `association=(associate)`
-* `build_association(attributes = {})`
-* `create_association(attributes = {})`
-* `create_association!(attributes = {})`
-* `reload_association`
-* `reset_association`
+##### Managing the Collection
-In all of these methods, `association` is replaced with the symbol passed as the first argument to `has_one`. For example, given the declaration:
+The `collection` method returns a Relation of all of the associated objects. If
+there are no associated objects, it returns an empty Relation.
```ruby
-class Supplier < ApplicationRecord
- has_one :account
-end
+@assemblies = @part.assemblies
```
-Each instance of the `Supplier` model will have these methods:
-
-* `account`
-* `account=`
-* `build_account`
-* `create_account`
-* `create_account!`
-* `reload_account`
-* `reset_account`
+The [`collection<<`][] method adds one or more objects to the collection by
+creating records in the join table.
-NOTE: When initializing a new `has_one` or `belongs_to` association you must use the `build_` prefix to build the association, rather than the `association.build` method that would be used for `has_many` or `has_and_belongs_to_many` associations. To create one, use the `create_` prefix.
+```ruby
+@part.assemblies << @assembly1
+```
-##### `association`
+NOTE: This method is aliased as `collection.concat` and `collection.push`.
-The `association` method returns the associated object, if any. If no associated object is found, it returns `nil`.
+The [`collection.delete`][] method removes one or more objects from the
+collection by deleting records in the join table. This does not destroy the
+objects.
```ruby
-@account = @supplier.account
+@part.assemblies.delete(@assembly1)
```
-If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), call `#reload_association` on the parent object.
+The [`collection.destroy`][] method removes one or more objects from the
+collection by deleting records in the join table. This does not destroy the
+objects.
```ruby
-@account = @supplier.reload_account
+@part.assemblies.destroy(@assembly1)
```
-To unload the cached version of the associated object—forcing the next access, if any, to query it from the database—call `#reset_association` on the parent object.
+The [`collection.clear`][] method removes every object from the collection by
+deleting the rows from the joining table. This does not destroy the associated
+objects.
-```ruby
-@supplier.reset_account
-```
-##### `association=(associate)`
+##### Assigning the Collection
-The `association=` method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from this object and setting the associated object's foreign key to the same value.
+The `collection=` method makes the collection contain only the supplied objects,
+by adding and deleting as appropriate. The changes are persisted to the
+database.
-```ruby
-@supplier.account = @account
-```
+The `collection_singular_ids=` method makes the collection contain only the
+objects identified by the supplied primary key values, by adding and deleting as
+appropriate. The changes are persisted to the database.
-##### `build_association(attributes = {})`
+##### Querying the Collection
-The `build_association` method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through its foreign key will be set, but the associated object will _not_ yet be saved.
+The `collection_singular_ids` method returns an array of the ids of the objects
+in the collection.
```ruby
-@account = @supplier.build_account(terms: "Net 30")
+@assembly_ids = @part.assembly_ids
```
-##### `create_association(attributes = {})`
+The [`collection.empty?`][] method returns `true` if the collection does not
+contain any associated objects.
+
+```html+erb
+<% if @part.assemblies.empty? %>
+ This part is not used in any assemblies
+<% end %>
+```
-The `create_association` method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through its foreign key will be set, and, once it passes all of the validations specified on the associated model, the associated object _will_ be saved.
+The [`collection.size`][] method returns the number of objects in the
+collection.
```ruby
-@account = @supplier.create_account(terms: "Net 30")
+@assembly_count = @part.assemblies.size
```
-##### `create_association!(attributes = {})`
-
-Does the same as `create_association` above, but raises `ActiveRecord::RecordInvalid` if the record is invalid.
+The [`collection.find`][] method finds objects within the collection's table.
-#### Options for `has_one`
+```ruby
+@assembly = @part.assemblies.find(1)
+```
-While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `has_one` association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:
+The [`collection.where`][] method finds objects within the collection based on
+the conditions supplied but the objects are loaded lazily meaning that the
+database is queried only when the object(s) are accessed.
```ruby
-class Supplier < ApplicationRecord
- has_one :account, class_name: "Billing", dependent: :nullify
-end
+@new_assemblies = @part.assemblies.where("created_at > ?", 2.days.ago)
```
-The [`has_one`][] association supports these options:
+The [`collection.exists?`][] method checks whether an object meeting the
+supplied conditions exists in the collection's table.
+
-* `:as`
-* `:autosave`
-* `:class_name`
-* `:dependent`
-* `:disable_joins`
-* `:ensuring_owner_was`
-* `:foreign_key`
-* `:inverse_of`
-* `:primary_key`
-* `:query_constraints`
-* `:required`
-* `:source`
-* `:source_type`
-* `:strict_loading`
-* `:through`
-* `:touch`
-* `:validate`
+##### Building and Creating Associated Objects
-##### `:as`
+The [`collection.build`][] method returns a new object of the associated type.
+This object will be instantiated from the passed attributes, and the link
+through the join table will be created, but the associated object will _not_ yet
+be saved.
-Setting the `:as` option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail [earlier in this guide](#polymorphic-associations).
+```ruby
+@assembly = @part.assemblies.build({ assembly_name: "Transmission housing" })
+```
-##### `:autosave`
+The [`collection.create`][] method returns a new object of the associated type.
+This object will be instantiated from the passed attributes, the link through
+the join table will be created, and, once it passes all of the validations
+specified on the associated model, the associated object _will_ be saved.
-If you set the `:autosave` option to `true`, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting `:autosave` to `false` is not the same as not setting the `:autosave` option. If the `:autosave` option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
+```ruby
+@assembly = @part.assemblies.create({ assembly_name: "Transmission housing" })
+```
-##### `:class_name`
+Does the same as `collection.create`, but raises `ActiveRecord::RecordInvalid`
+if the record is invalid.
-If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if a supplier has an account, but the actual name of the model containing accounts is `Billing`, you'd set things up this way:
+The [`collection.reload`][] method returns a Relation of all of the associated
+objects, forcing a database read. If there are no associated objects, it returns
+an empty Relation.
```ruby
-class Supplier < ApplicationRecord
- has_one :account, class_name: "Billing"
-end
+@assemblies = @part.assemblies.reload
```
-##### `:dependent`
+##### When are Objects Saved?
-Controls what happens to the associated object when its owner is destroyed:
+When you assign an object to a `has_and_belongs_to_many` association, that
+object is automatically saved (in order to update the join table). If you assign
+multiple objects in one statement, then they are all saved.
-* `:destroy` causes the associated object to also be destroyed
-* `:delete` causes the associated object to be deleted directly from the database (so callbacks will not execute)
-* `:destroy_async`: when the object is destroyed, an `ActiveRecord::DestroyAssociationAsyncJob` job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work. Do not use this option if the association is backed by foreign key constraints in your database. The foreign key constraint actions will occur inside the same transaction that deletes its owner.
-* `:nullify` causes the foreign key to be set to `NULL`. Polymorphic type column is also nullified on polymorphic associations. Callbacks are not executed.
-* `:restrict_with_exception` causes an `ActiveRecord::DeleteRestrictionError` exception to be raised if there is an associated record
-* `:restrict_with_error` causes an error to be added to the owner if there is an associated object
+If any of these saves fails due to validation errors, then the assignment
+statement returns `false` and the assignment itself is cancelled.
-It's necessary not to set or leave `:nullify` option for those associations
-that have `NOT NULL` database constraints. If you don't set `dependent` to
-destroy such associations you won't be able to change the associated object
-because the initial associated object's foreign key will be set to the
-unallowed `NULL` value.
+If the parent object (the one declaring the `has_and_belongs_to_many`
+association) is unsaved (that is, `new_record?` returns `true`) then the child
+objects are not saved when they are added. All unsaved members of the
+association will automatically be saved when the parent is saved.
-##### `:disable_joins`
+If you want to assign an object to a `has_and_belongs_to_many` association
+without saving the object, use the `collection.build` method.
-Specifies whether joins should be skipped for an association. If set to `true`, two or more queries will be generated. Note that in some cases, if order or limit is applied, it will be done in-memory due to database limitations. This option is only applicable on `has_one :through` associations as `has_one` alone does not perform a join.
+Choosing an Association
+-----------------------
-##### `:foreign_key`
+### `belongs_to` vs `has_one`
-By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly:
+If you want to set up a one-to-one relationship between two models, you can
+choose between a `belongs_to` and a `has_one` association. How do you know which
+one to choose?
-```ruby
-class Supplier < ApplicationRecord
- has_one :account, foreign_key: "supp_id"
-end
-```
-TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
+The distinction lies in the placement of the foreign key, which goes on the
+table of the class declaring the `belongs_to` association. However, it’s
+essential to understand the semantics to determine the correct associations:
+
+- `belongs_to`: This association indicates that the current model contains the
+ foreign key and is a child in the relationship. It references another model,
+ implying that each instance of this model is linked to one instance of the
+ other model.
+- `has_one`: This association indicates that the current model is the parent in
+ the relationship, and it owns one instance of the other model.
-##### `:inverse_of`
+For example, consider a scenario with suppliers and their accounts. It makes
+more sense to say that a supplier has/owns an account (where the supplier is the
+parent) rather than an account has/owns a supplier. Therefore, the correct
+associations would be:
-The `:inverse_of` option specifies the name of the `belongs_to` association that is the inverse of this association.
-See the [bi-directional association](#bi-directional-associations) section for more details.
+- A supplier has one account.
+- An account belongs to one supplier.
+
+Here is how you can define these associations in Rails:
```ruby
class Supplier < ApplicationRecord
- has_one :account, inverse_of: :supplier
+ has_one :account
end
class Account < ApplicationRecord
- belongs_to :supplier, inverse_of: :account
+ belongs_to :supplier
end
```
-##### `:primary_key`
-
-By convention, Rails assumes that the column used to hold the primary key of this model is `id`. You can override this and explicitly specify the primary key with the `:primary_key` option.
+To implement these associations, you'll need to create the corresponding
+database tables and set up the foreign key. Here's an example migration:
-##### `:query_constraints`
-
-Serves as a composite foreign key. Defines the list of columns to be used to query the associated object. This is an optional option. By default Rails will attempt to derive the value automatically. When the value is set the Array size must match associated model’s primary key or query_constraints size.
+```ruby
+class CreateSuppliers < ActiveRecord::Migration[7.2]
+ def change
+ create_table :suppliers do |t|
+ t.string :name
+ t.timestamps
+ end
-##### `:required`
+ create_table :accounts do |t|
+ t.belongs_to :supplier_id
+ t.string :account_number
+ t.timestamps
+ end
-When set to `true`, the association will also have its presence validated. This will validate the association itself, not the id. You can use `:inverse_of` to avoid an extra query during validation.
+ add_index :accounts, :supplier_id
+ end
+end
+```
-##### `:source`
+Remember that the foreign key goes on the table of the class declaring the
+belongs_to association. In this case the `account` table.
-The `:source` option specifies the source association name for a `has_one :through` association.
+### `has_many :through` vs `has_and_belongs_to_many`
-##### `:source_type`
+Rails offers two different ways to declare a many-to-many relationship between
+models: `has_many :through` and `has_and_belongs_to_many`. Understanding the
+differences and use cases for each can help you choose the best approach for
+your application's needs.
-The `:source_type` option specifies the source association type for a `has_one :through` association that proceeds through a polymorphic association.
+The `has_many :through` association sets up a many-to-many relationship through
+an intermediary model (also known as a join model). This approach is more
+flexible and allows you to add validations, callbacks, and extra attributes to
+the join model. The join table needs a `primary_key` (or a [composite primary
+key](active_record_composite_primary_keys.html)).
```ruby
-class Author < ApplicationRecord
- has_one :book
- has_one :hardback, through: :book, source: :format, source_type: "Hardback"
- has_one :dust_jacket, through: :hardback
+class Assembly < ApplicationRecord
+ has_many :manifests
+ has_many :parts, through: :manifests
end
-class Book < ApplicationRecord
- belongs_to :format, polymorphic: true
+class Manifest < ApplicationRecord
+ belongs_to :assembly
+ belongs_to :part
end
-class Paperback < ApplicationRecord; end
-
-class Hardback < ApplicationRecord
- has_one :dust_jacket
+class Part < ApplicationRecord
+ has_many :manifests
+ has_many :assemblies, through: :manifests
end
-
-class DustJacket < ApplicationRecord; end
```
-##### `:strict_loading`
-
-Enforces strict loading every time the associated record is loaded through this association.
+You'd use `has_many :through` when:
-##### `:through`
+- You need to add extra attributes or methods to the join table.
+- You require [validations](active_record_validations.html) or
+ [callbacks](active_record_callbacks.html) on the join model.
+- The join table should be treated as an independent entity with its own
+ behavior.
-The `:through` option specifies a join model through which to perform the query. `has_one :through` associations were discussed in detail [earlier in this guide](#the-has-one-through-association).
-
-##### `:touch`
-
-If you set the `:touch` option to `true`, then the `updated_at` or `updated_on` timestamp on the associated object will be set to the current time whenever this object is saved or destroyed:
+The `has_and_belongs_to_many` association allows you to create a many-to-many
+relationship directly between two models without needing an intermediary model.
+This method is straightforward and is suitable for simple associations where no
+additional attributes or behaviors are required on the join table. For
+`has_and_belongs_to_many` associations, you'll need to create a join table
+without a primary key.
```ruby
-class Supplier < ApplicationRecord
- has_one :account, touch: true
+class Assembly < ApplicationRecord
+ has_and_belongs_to_many :parts
end
-class Account < ApplicationRecord
- belongs_to :supplier
+class Part < ApplicationRecord
+ has_and_belongs_to_many :assemblies
end
```
-In this case, saving or destroying a supplier will update the timestamp on the associated account. You can also specify a particular timestamp attribute to update:
+You'd use `has_and_belongs_to_many` when:
-```ruby
-class Supplier < ApplicationRecord
- has_one :account, touch: :suppliers_updated_at
-end
-```
+- The association is simple and does not require additional attributes or
+ behaviors on the join table.
+- You do not need validations, callbacks, or extra methods on the join table.
-##### `:validate`
+Advanced Associations
+-------------------------
-If you set the `:validate` option to `true`, then new associated objects will be validated whenever you save this object. By default, this is `false`: new associated objects will not be validated when this object is saved.
+### Polymorphic Associations
-#### Scopes for `has_one`
+A slightly more advanced twist on associations is the _polymorphic association_.
+Polymorphic associations in Rails allow a model to belong to multiple other
+models through a single association. This can be particularly useful when you
+have a model that needs to be linked to different types of models.
-There may be times when you wish to customize the query used by `has_one`. Such customizations can be achieved via a scope block. For example:
+For instance, imagine you have a `Picture` model that can belong to **either**
+an `Employee` or a `Product`, because each of these can have a profile picture.
+Here's how this could be declared:
```ruby
-class Supplier < ApplicationRecord
- has_one :account, -> { where active: true }
+class Picture < ApplicationRecord
+ belongs_to :imageable, polymorphic: true
end
-```
-
-You can use any of the standard [querying methods](active_record_querying.html) inside the scope block. The following ones are discussed below:
-* `where`
-* `includes`
-* `readonly`
-* `select`
-
-##### `where`
-
-The `where` method lets you specify the conditions that the associated object must meet.
+class Employee < ApplicationRecord
+ has_many :pictures, as: :imageable
+end
-```ruby
-class Supplier < ApplicationRecord
- has_one :account, -> { where "confirmed = 1" }
+class Product < ApplicationRecord
+ has_many :pictures, as: :imageable
end
```
-##### `includes`
+
-You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
+In the context above, `imageable` is a name chosen for the association. It's a
+symbolic name that represents the polymorphic association between the `Picture`
+model and other models such as `Employee` and `Product`. The important thing is
+to use the same name (`imageable`) consistently across all associated models to
+establish the polymorphic association correctly.
-```ruby
-class Supplier < ApplicationRecord
- has_one :account
-end
+When you declare `belongs_to :imageable, polymorphic: true` in the `Picture`
+model, you're saying that a `Picture` can belong to any model (like `Employee`
+or `Product`) through this association.
-class Account < ApplicationRecord
- belongs_to :supplier
- belongs_to :representative
-end
+You can think of a polymorphic `belongs_to` declaration as setting up an
+interface that any other model can use. This allows you to retrieve a collection
+of pictures from an instance of the `Employee` model using `@employee.pictures`.
+Similarly, you can retrieve a collection of pictures from an instance of the
+`Product` model using `@product.pictures`.
-class Representative < ApplicationRecord
- has_many :accounts
-end
-```
+Additionally, if you have an instance of the `Picture` model, you can get its
+parent via `@picture.imageable`, which could be an `Employee` or a `Product`.
-If you frequently retrieve representatives directly from suppliers (`@supplier.account.representative`), then you can make your code somewhat more efficient by including representatives in the association from suppliers to accounts:
+To setup a polymorphic association manually you would need to declare both a
+foreign key column (`imageable_id`) and a type column (`imageable_type`) in the
+model:
```ruby
-class Supplier < ApplicationRecord
- has_one :account, -> { includes :representative }
-end
-
-class Account < ApplicationRecord
- belongs_to :supplier
- belongs_to :representative
-end
+class CreatePictures < ActiveRecord::Migration[7.2]
+ def change
+ create_table :pictures do |t|
+ t.string :name
+ t.bigint :imageable_id
+ t.string :imageable_type
+ t.timestamps
+ end
-class Representative < ApplicationRecord
- has_many :accounts
+ add_index :pictures, [:imageable_type, :imageable_id]
+ end
end
```
-##### `readonly`
-
-If you use the `readonly` method, then the associated object will be read-only when retrieved via the association.
-
-##### `select`
+In our example, `imageable_id` could be the ID of either an `Employee` or a
+`Product`, and `imageable_type` is the name of the associated model's class, so
+either `Employee` or `Product`.
-The `select` method lets you override the SQL `SELECT` clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.
-
-#### Do Any Associated Objects Exist?
-
-You can see if any associated objects exist by using the `association.nil?` method:
+While creating the polymorphic association manually is acceptable, it is instead
+recommended to use `t.references` or its alias `t.belong_to` and specify
+`polymorphic: true` so that Rails knows that the association is polymorphic, and
+it automatically adds both the foreign key and type columns to the table.
```ruby
-if @supplier.account.nil?
- @msg = "No account found for this supplier"
+class CreatePictures < ActiveRecord::Migration[7.2]
+ def change
+ create_table :pictures do |t|
+ t.string :name
+ t.belongs_to :imageable, polymorphic: true
+ t.timestamps
+ end
+ end
end
```
-#### When are Objects Saved?
+WARNING: Since polymorphic associations rely on storing class names in the
+database, that data must remain synchronized with the class name used by the
+Ruby code. When renaming a class, make sure to update the data in the
+polymorphic type column.
-When you assign an object to a `has_one` association, that object is automatically saved (in order to update its foreign key). In addition, any object being replaced is also automatically saved, because its foreign key will change too.
+For example, if you change the class name from `Product` to `Item` then you'd
+need to run a migration script to update the `imageable_type` column in the
+`pictures` table (or whichever table is affected) with the new class name.
+Additionally, you'll need to update any other references to the class name
+throughout your application code to reflect the change.
-If either of these saves fails due to validation errors, then the assignment statement returns `false` and the assignment itself is cancelled.
+### Models with Composite Primary Keys
-If the parent object (the one declaring the `has_one` association) is unsaved (that is, `new_record?` returns `true`) then the child objects are not saved. They will automatically be saved when the parent object is saved.
+Rails can often infer primary key-foreign key relationships between associated
+models, but when dealing with composite primary keys, Rails typically defaults
+to using only part of the composite key, often the id column, unless explicitly
+instructed otherwise.
-If you want to assign an object to a `has_one` association without saving the object, use the `build_association` method.
+If you're working with composite primary keys in your Rails models and need to
+ensure the correct handling of associations, please refer to the [Associations
+section of the Composite Primary Keys
+guide](active_record_composite_primary_keys#associations-between-models-with-composite-primary-keys).
+This section provides comprehensive guidance on setting up and using
+associations with composite primary keys in Rails, including how to specify
+composite foreign keys when necessary.
-### `has_many` Association Reference
+### Self Joins
-The `has_many` association creates a one-to-many relationship with another model. In database terms, this association says that the other class will have a foreign key that refers to instances of this class.
+A self-join is a regular join, but the table is joined with itself. This is
+useful in situations where there is a hierarchical relationship within a single
+table. A common example is an employee management system where an employee can
+have a manager, and that manager is also an employee.
-#### Methods Added by `has_many`
+Consider an organization where employees can be managers of other employees. We
+want to track this relationship using a single `employees` table.
-When you declare a `has_many` association, the declaring class automatically gains 17 methods related to the association:
+In your Rails model, you define the `Employee` class to reflect these
+relationships:
-* `collection`
-* [`collection<<(object, ...)`][`collection<<`]
-* [`collection.delete(object, ...)`][`collection.delete`]
-* [`collection.destroy(object, ...)`][`collection.destroy`]
-* `collection=(objects)`
-* `collection_singular_ids`
-* `collection_singular_ids=(ids)`
-* [`collection.clear`][]
-* [`collection.empty?`][]
-* [`collection.size`][]
-* [`collection.find(...)`][`collection.find`]
-* [`collection.where(...)`][`collection.where`]
-* [`collection.exists?(...)`][`collection.exists?`]
-* [`collection.build(attributes = {})`][`collection.build`]
-* [`collection.create(attributes = {})`][`collection.create`]
-* [`collection.create!(attributes = {})`][`collection.create!`]
-* [`collection.reload`][]
+```ruby
+class Employee < ApplicationRecord
+ # an employee can have many subordinates.
+ has_many :subordinates, class_name: "Employee", foreign_key: "manager_id"
+
+ # an employee can have one manager.
+ belongs_to :manager, class_name: "Employee", optional: true
+end
+```
+
+`has_many :subordinates` sets up a one-to-many relationship where an employee
+can have many subordinates. Here, we specify that the related model is also
+`Employee` (`class_name: "Employee"`) and the foreign key used to identify the
+manager is `manager_id`.
+`belongs_to :manager` sets up a one-to-one relationship where an employee can
+belong to one manager. Again, we specify the related model as `Employee`.
-In all of these methods, `collection` is replaced with the symbol passed as the first argument to `has_many`, and `collection_singular` is replaced with the singularized version of that symbol. For example, given the declaration:
+To support this relationship, we need to add a `manager_id` column to the
+`employees` table. This column references the `id` of another employee (the
+manager).
```ruby
-class Author < ApplicationRecord
- has_many :books
+class CreateEmployees < ActiveRecord::Migration[7.2]
+ def change
+ create_table :employees do |t|
+ # Add a belongs_to reference to the manager, which is an employee.
+ t.belongs_to :manager, foreign_key: { to_table: :employees }
+ t.timestamps
+ end
+ end
end
```
-Each instance of the `Author` model will have these methods:
-
-```
-books
-books<<(object, ...)
-books.delete(object, ...)
-books.destroy(object, ...)
-books=(objects)
-book_ids
-book_ids=(ids)
-books.clear
-books.empty?
-books.size
-books.find(...)
-books.where(...)
-books.exists?(...)
-books.build(attributes = {}, ...)
-books.create(attributes = {})
-books.create!(attributes = {})
-books.reload
-```
+- `t.belongs_to :manager` adds a `manager_id` column to the `employees` table.
+- `foreign_key: { to_table: :employees }` ensures that the `manager_id` column
+ references the `id` column of the `employees` table.
-[`collection<<`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-3C-3C
-[`collection.build`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-build
-[`collection.clear`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-clear
-[`collection.create`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-create
-[`collection.create!`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-create-21
-[`collection.delete`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-delete
-[`collection.destroy`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-destroy
-[`collection.empty?`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-empty-3F
-[`collection.exists?`]: https://api.rubyonrails.org/classes/ActiveRecord/FinderMethods.html#method-i-exists-3F
-[`collection.find`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-find
-[`collection.reload`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-reload
-[`collection.size`]: https://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html#method-i-size
-[`collection.where`]: https://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#method-i-where
+NOTE: The `to_table` option passed to `foreign_key` and more are explained in
+[`SchemaStatements#add_reference`][connection.add_reference].
-##### `collection`
+With this setup, you can easily access an employee's subordinates and manager in
+your Rails application.
-The `collection` method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation.
+To get an employee's subordinates:
```ruby
-@books = @author.books
+employee = Employee.find(1)
+subordinates = employee.subordinates
```
-##### `collection<<(object, ...)`
-
-The [`collection<<`][] method adds one or more objects to the collection by setting their foreign keys to the primary key of the calling model.
+To get an employee's manager:
```ruby
-@author.books << @book1
+manager = employee.manager
```
-##### `collection.delete(object, ...)`
+[connection.add_reference]:
+ https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_reference
-The [`collection.delete`][] method removes one or more objects from the collection by setting their foreign keys to `NULL`.
+Single Table Inheritance (STI)
+------------------------------
-```ruby
-@author.books.delete(@book1)
-```
+Single Table Inheritance (STI) is a pattern in Rails that allows multiple models
+to be stored in a single database table. This is useful when you have different
+types of entities that share common attributes and behavior but also have
+specific behaviors.
-WARNING: Additionally, objects will be destroyed if they're associated with `dependent: :destroy`, and deleted if they're associated with `dependent: :delete_all`.
+For example, suppose we have `Car`, `Motorcycle`, and `Bicycle` models. These
+models will share fields like `color` and `price`, but each will have unique
+behaviors. They will also each have their own controller.
-##### `collection.destroy(object, ...)`
+### Generating the Base Vehicle Model
-The [`collection.destroy`][] method removes one or more objects from the collection by running `destroy` on each object.
+First, we generate the base `Vehicle` model with shared fields:
-```ruby
-@author.books.destroy(@book1)
+```bash
+$ bin/rails generate model vehicle type:string color:string price:decimal{10.2}
```
-WARNING: Objects will _always_ be removed from the database, ignoring the `:dependent` option.
+Here, the `type` field is crucial for STI as it stores the model name (`Car`,
+`Motorcycle`, or `Bicycle`). STI requires this field to differentiate between
+the different models stored in the same table.
+
+### Generating Child Models
-##### `collection=(objects)`
+Next, we generate the `Car`, `Motorcycle`, and `Bicycle` models that inherit
+from Vehicle. These models won't have their own tables; instead, they will use
+the `vehicles` table.
-The `collection=` method makes the collection contain only the supplied objects, by adding and deleting as appropriate. The changes are persisted to the database.
+To generate the`Car` model:
+
+```bash
+$ bin/rails generate model car --parent=Vehicle
+```
-##### `collection_singular_ids`
+For this, we can use the `--parent=PARENT` option, which will generate a model
+that inherits from the specified parent and without equivalent migration (since
+the table already exists).
-The `collection_singular_ids` method returns an array of the ids of the objects in the collection.
+This generates a `Car` model that inherits from `Vehicle`:
```ruby
-@book_ids = @author.book_ids
+class Car < Vehicle
+end
```
-##### `collection_singular_ids=(ids)`
+This means that all behavior added to Vehicle is available for Car too, as
+associations, public methods, etc. Creating a car will save it in the `vehicles`
+table with "Car" as the `type` field:
-The `collection_singular_ids=` method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate. The changes are persisted to the database.
+Repeat the same process for `Motorcycle` and `Bicycle`.
-##### `collection.clear`
+### Creating Records
-The [`collection.clear`][] method removes all objects from the collection according to the strategy specified by the `dependent` option. If no option is given, it follows the default strategy. The default strategy for `has_many :through` associations is `delete_all`, and for `has_many` associations is to set the foreign keys to `NULL`.
+Creating a record for `Car`:
```ruby
-@author.books.clear
+Car.create(color: 'Red', price: 10000)
```
-WARNING: Objects will be deleted if they're associated with `dependent: :destroy` or `dependent: :destroy_async`,
-just like `dependent: :delete_all`.
-
-##### `collection.empty?`
-
-The [`collection.empty?`][] method returns `true` if the collection does not contain any associated objects.
+This will generate the following SQL:
-```erb
-<% if @author.books.empty? %>
- No Books Found
-<% end %>
+```sql
+INSERT INTO "vehicles" ("type", "color", "price") VALUES ('Car', 'Red', 10000)
```
-##### `collection.size`
+### Querying Records
-The [`collection.size`][] method returns the number of objects in the collection.
+Querying car records will search only for vehicles that are cars:
```ruby
-@book_count = @author.books.size
+Car.all
```
-##### `collection.find(...)`
-
-The [`collection.find`][] method finds objects within the collection's table.
+will run a query like:
-```ruby
-@available_book = @author.books.find(1)
+```sql
+SELECT "vehicles".* FROM "vehicles" WHERE "vehicles"."type" IN ('Car')
```
-##### `collection.where(...)`
+### Adding Specific Behavior
-The [`collection.where`][] method finds objects within the collection based on the conditions supplied but the objects are loaded lazily meaning that the database is queried only when the object(s) are accessed.
+You can add specific behavior or methods to the child models. For example,
+adding a method to the `Car` model:
```ruby
-@available_books = @author.books.where(available: true) # No query yet
-@available_book = @available_books.first # Now the database will be queried
+class Car < Vehicle
+ def honk
+ 'Beep Beep'
+ end
+end
```
-##### `collection.exists?(...)`
+Now you can call the `honk` method on a `Car` instance:
-The [`collection.exists?`][] method checks whether an object meeting the supplied
-conditions exists in the collection's table.
+```ruby
+car = Car.first
+car.honk
+# => 'Beep Beep'
+```
-##### `collection.build(attributes = {})`
+### Controllers
-The [`collection.build`][] method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, and the link through their foreign key will be created, but the associated objects will _not_ yet be saved.
+Each model can have its own controller. For example, the `CarsController`:
```ruby
-@book = @author.books.build(published_at: Time.now,
- book_number: "A12345")
+# app/controllers/cars_controller.rb
-@books = @author.books.build([
- { published_at: Time.now, book_number: "A12346" },
- { published_at: Time.now, book_number: "A12347" }
-])
+class CarsController < ApplicationController
+ def index
+ @cars = Car.all
+ end
+end
```
-##### `collection.create(attributes = {})`
+### Overriding the inheritance column
-The [`collection.create`][] method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, the link through its foreign key will be created, and, once it passes all of the validations specified on the associated model, the associated object _will_ be saved.
+There may be cases (like when working with a legacy database) where you need to
+override the name of the inheritance column. This can be achieved with the
+[inheritance_column][] method.
```ruby
-@book = @author.books.create(published_at: Time.now,
- book_number: "A12345")
+# Schema: vehicles[ id, kind, created_at, updated_at ]
+class Vehicle < ApplicationRecord
+ self.inheritance_column = "kind"
+end
-@books = @author.books.create([
- { published_at: Time.now, book_number: "A12346" },
- { published_at: Time.now, book_number: "A12347" }
-])
+class Car < Vehicle
+end
+
+Car.create
+# => #
```
-##### `collection.create!(attributes = {})`
+In this setup, Rails will use the `kind` column to store the model type,
+allowing STI to function correctly with the custom column name.
-Does the same as `collection.create` above, but raises `ActiveRecord::RecordInvalid` if the record is invalid.
+### Disabling the inheritance column
-##### `collection.reload`
+There may be cases (like when working with a legacy database) where you need to
+disable Single Table Inheritance altogether. If you don't disable STI properly,
+you might encounter an [`ActiveRecord::SubclassNotFound`][] error.
-The [`collection.reload`][] method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation.
+To disable STI, you can set the [inheritance_column][] to `nil`.
```ruby
-@books = @author.books.reload
+# Schema: vehicles[ id, type, created_at, updated_at ]
+class Vehicle < ApplicationRecord
+ self.inheritance_column = nil
+end
+
+Vehicle.create!(type: "Car")
+# => #
```
-#### Options for `has_many`
+In this configuration, Rails will treat the type column as a normal attribute
+and will not use it for STI purposes. This is useful if you need to work with a
+legacy schema that does not follow the STI pattern.
-While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `has_many` association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:
+These adjustments provide flexibility when integrating Rails with existing
+databases or when specific customization is required for your models.
-```ruby
-class Author < ApplicationRecord
- has_many :books, dependent: :delete_all, validate: false
-end
-```
+[inheritance_column]:
+ https://api.rubyonrails.org/classes/ActiveRecord/ModelSchema.html#method-c-inheritance_column
+[`ActiveRecord::SubclassNotFound`]:
+ https://api.rubyonrails.org/classes/ActiveRecord/SubclassNotFound.html
-The [`has_many`][] association supports these options:
+### Considerations
-* `:as`
-* `:autosave`
-* `:class_name`
-* `:counter_cache`
-* `:dependent`
-* `:disable_joins`
-* `:ensuring_owner_was`
-* `:extend`
-* `:foreign_key`
-* `:foreign_type`
-* `:inverse_of`
-* `:primary_key`
-* `:query_constraints`
-* `:source`
-* `:source_type`
-* `:strict_loading`
-* `:through`
-* `:validate`
+[`Single Table Inheritance (STI)`](#single-table-inheritance-sti) works best
+when there is little difference between subclasses and their attributes, but it
+includes all attributes of all subclasses in a single table.
-##### `:as`
+A disadvantage of this approach is that it can result in table bloat, as the
+table will include attributes specific to each subclass, even if they aren't
+used by others. This can be solved by using [`Delegated
+Types`](#delegated-types).
-Setting the `:as` option indicates that this is a polymorphic association, as discussed [earlier in this guide](#polymorphic-associations).
+Additionally, if you’re using [polymorphic
+associations](#polymorphic-associations), where a model can belong to more than
+one other model via a type and an ID, it could become complex to maintain
+referential integrity because the association logic must handle different types
+correctly.
-##### `:autosave`
+Finally, if you have specific data integrity checks or validations that differ
+between subclasses, you need to ensure these are correctly handled by Rails or
+the database, especially when setting up foreign key constraints.
-If you set the `:autosave` option to `true`, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting `:autosave` to `false` is not the same as not setting the `:autosave` option. If the `:autosave` option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
+Delegated Types
+----------------
-##### `:class_name`
+Delegated types solves the [`Single Table Inheritance
+(STI)`](#single-table-inheritance-sti) problem of table bloat via
+`delegated_type`. This approach allows us to store shared attributes in a
+superclass table and have separate tables for subclass-specific attributes.
-If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if an author has many books, but the actual name of the model containing books is `Transaction`, you'd set things up this way:
+### Setting up Delegated Types
-```ruby
-class Author < ApplicationRecord
- has_many :books, class_name: "Transaction"
-end
-```
+To use delegated types, we need to model our data as follows:
-##### `:counter_cache`
+* There is a superclass that stores shared attributes among all subclasses in
+ its table.
+* Each subclass must inherit from the superclass, and will have a separate table
+ for any additional attributes specific to it.
-This option can be used to configure a custom named `:counter_cache`. You only need this option when you customized the name of your `:counter_cache` on the [belongs_to association](#options-for-belongs-to).
+This eliminates the need to define attributes in a single table that are
+unintentionally shared among all subclasses.
-##### `:dependent`
+### Generating Models
-Controls what happens to the associated objects when their owner is destroyed:
+In order to apply this to our example above, we need to regenerate our models.
-* `:destroy` causes all the associated objects to also be destroyed
-* `:delete_all` causes all the associated objects to be deleted directly from the database (so callbacks will not execute)
-* `:destroy_async`: when the object is destroyed, an `ActiveRecord::DestroyAssociationAsyncJob` job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work.
-* `:nullify` causes the foreign key to be set to `NULL`. Polymorphic type column is also nullified on polymorphic associations. Callbacks are not executed.
-* `:restrict_with_exception` causes an `ActiveRecord::DeleteRestrictionError` exception to be raised if there are any associated records
-* `:restrict_with_error` causes an error to be added to the owner if there are any associated objects
+First, let's generate the base `Entry` model which will act as our superclass:
-The `:destroy` and `:delete_all` options also affect the semantics of the `collection.delete` and `collection=` methods by causing them to destroy associated objects when they are removed from the collection.
+```bash
+$ bin/rails generate model entry entryable_type:string entryable_id:integer
+```
-##### `:disable_joins`
+Then, we will generate new `Message` and `Comment` models for delegation:
-Specifies whether joins should be skipped for an association. If set to true, two or more queries will be generated. Note that in some cases, if order or limit is applied, it will be done in-memory due to database limitations. This option is only applicable on `has_many :through associations` as has_many alone do not perform a join.
+```bash
+$ bin/rails generate model message subject:string body:string
+$ bin/rails generate model comment content:string
+```
-##### `:ensuring_owner_was`
+After running the generators, our models should look like this:
-Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.
+```ruby
+# Schema: entries[ id, entryable_type, entryable_id, created_at, updated_at ]
+class Entry < ApplicationRecord
+end
-##### `:extend`
+# Schema: messages[ id, subject, body, created_at, updated_at ]
+class Message < ApplicationRecord
+end
-Specifies a module or array of modules that will be extended into the association object returned. Useful for defining methods on associations, especially when they should be shared between multiple association objects.
+# Schema: comments[ id, content, created_at, updated_at ]
+class Comment < ApplicationRecord
+end
+```
-##### `:foreign_key`
+### Declaring `delegated_type`
-By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly:
+First, declare a `delegated_type` in the superclass `Entry`.
```ruby
-class Author < ApplicationRecord
- has_many :books, foreign_key: "cust_id"
+class Entry < ApplicationRecord
+ delegated_type :entryable, types: %w[ Message Comment ], dependent: :destroy
end
```
-TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
+The `entryable` parameter specifies the field to use for delegation, and include
+the types `Message` and `Comment` as the delegate classes. The `entryable_type`
+and `entryable_id` fields store the subclass name and the record ID of the
+delegate subclass, respectively.
+
+### Defining the `Entryable` Module
-##### `:foreign_type`
+Next, define a module to implement those delegated types by declaring the `as:
+:entryable` parameter in the `has_one` association.
-Specify the column used to store the associated object’s type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “`as`” option with a “`_type`” suffix. So a class that defines a `has_many :tags, as: :taggable` association will use “`taggable_type`” as the default `:foreign_type`.
+```ruby
+module Entryable
+ extend ActiveSupport::Concern
-##### `:inverse_of`
+ included do
+ has_one :entry, as: :entryable, touch: true
+ end
+end
+```
-The `:inverse_of` option specifies the name of the `belongs_to` association that is the inverse of this association.
-See the [bi-directional association](#bi-directional-associations) section for more details.
+Include the created module in your subclass:
```ruby
-class Author < ApplicationRecord
- has_many :books, inverse_of: :author
+class Message < ApplicationRecord
+ include Entryable
end
-class Book < ApplicationRecord
- belongs_to :author, inverse_of: :books
+class Comment < ApplicationRecord
+ include Entryable
end
```
-##### `:primary_key`
+With this definition complete, our `Entry` delegator now provides the following
+methods:
-By convention, Rails assumes that the column used to hold the primary key of the association is `id`. You can override this and explicitly specify the primary key with the `:primary_key` option.
+| Method | Return |
+| ----------------------- | ------------------------------------------------------------------------------- |
+| `Entry.entryable_types` | ["Message", "Comment"] |
+| `Entry#entryable_class` | Message or Comment |
+| `Entry#entryable_name` | "message" or "comment" |
+| `Entry.messages` | `Entry.where(entryable_type: "Message")` |
+| `Entry#message?` | Returns true when `entryable_type == "Message"` |
+| `Entry#message` | Returns the message record, when `entryable_type == "Message"`, otherwise `nil` |
+| `Entry#message_id` | Returns `entryable_id`, when `entryable_type == "Message"`, otherwise `nil` |
+| `Entry.comments` | `Entry.where(entryable_type: "Comment")` |
+| `Entry#comment?` | Returns true when `entryable_type == "Comment"` |
+| `Entry#comment` | Returns the comment record, when `entryable_type == "Comment"`, otherwise `nil` |
+| `Entry#comment_id` | Returns entryable_id, when `entryable_type == "Comment"`, otherwise `nil` |
-Let's say the `users` table has `id` as the primary_key but it also
-has a `guid` column. The requirement is that the `todos` table should
-hold the `guid` column value as the foreign key and not `id`
-value. This can be achieved like this:
+### Object creation
+
+When creating a new `Entry` object, we can specify the `entryable` subclass at
+the same time.
```ruby
-class User < ApplicationRecord
- has_many :todos, primary_key: :guid
-end
+Entry.create! entryable: Message.new(subject: "hello!")
```
-Now if we execute `@todo = @user.todos.create` then the `@todo`
-record's `user_id` value will be the `guid` value of `@user`.
+### Adding further delegation
+
+We can enhance our `Entry` delegator by defining `delegate` and using
+polymorphism on the subclasses. For example, to delegate the `title` method from
+`Entry` to it's subclasses:
+
+```ruby
+class Entry < ApplicationRecord
+ delegated_type :entryable, types: %w[ Message Comment ]
+ delegate :title, to: :entryable
+end
+
+class Message < ApplicationRecord
+ include Entryable
+
+ def title
+ subject
+ end
+end
+
+class Comment < ApplicationRecord
+ include Entryable
+
+ def title
+ content.truncate(20)
+ end
+end
+```
-##### `:query_constraints`
+This setup allows `Entry` to delegate the `title` method to its subclasses,
+where `Message` uses `subject` and `Comment` uses a truncated version of
+`content`.
-Serves as a composite foreign key. Defines the list of columns to be used to query the associated object. This is an optional option. By default Rails will attempt to derive the value automatically. When the value is set the Array size must match associated model’s primary key or `query_constraints` size.
+Tips, Tricks, and Warnings
+--------------------------
-##### `:source`
+Here are a few things you should know to make efficient use of Active Record
+associations in your Rails applications:
-The `:source` option specifies the source association name for a `has_many :through` association. You only need to use this option if the name of the source association cannot be automatically inferred from the association name.
+* Controlling caching
+* Avoiding name collisions
+* Updating the schema
+* Controlling association scope
+* Bi-directional associations
-##### `:source_type`
+### Controlling Association Caching
-The `:source_type` option specifies the source association type for a `has_many :through` association that proceeds through a polymorphic association.
+All of the association methods are built around caching, which keeps the result
+of loaded associations for further operations. The cache is even shared across
+methods. For example:
```ruby
-class Author < ApplicationRecord
- has_many :books
- has_many :paperbacks, through: :books, source: :format, source_type: "Paperback"
-end
+# retrieves books from the database
+author.books.load
-class Book < ApplicationRecord
- belongs_to :format, polymorphic: true
-end
+# uses the cached copy of books
+author.books.size
-class Hardback < ApplicationRecord; end
-class Paperback < ApplicationRecord; end
+# uses the cached copy of books
+author.books.empty?
```
-##### `:strict_loading`
+NOTE: When we use `author.books`, the data is not immediately loaded from the
+database. Instead, it sets up a query that will be executed when you actually
+try to use the data, for example, by calling methods that require data like
+each, size, empty?, etc. By calling `author.books.load`, before calling other
+methods which use the data, you explicitly trigger the query to load the data
+from the database immediately. This is useful if you know you will need the data
+and want to avoid the potential performance overhead of multiple queries being
+triggered as you work with the association.
+
+But what if you want to reload the cache, because data might have been changed
+by some other part of the application? Just call
+[`reload`](https://api.rubyonrails.org/classes/ActiveRecord/Relation.html#method-i-reload)
+on the association:
+
+```ruby
+# retrieves books from the database
+author.books.load
+
+# uses the cached copy of books
+author.books.size
-When set to true, enforces strict loading every time the associated record is loaded through this association.
+# discards the cached copy of books and goes back to the database
+author.books.reload.empty?
+```
-##### `:through`
+### Avoiding Name Collisions
-The `:through` option specifies a join model through which to perform the query. `has_many :through` associations provide a way to implement many-to-many relationships, as discussed [earlier in this guide](#the-has-many-through-association).
+When creating associations in Ruby on Rails models, it's important to avoid
+using names that are already used for instance methods of `ActiveRecord::Base`.
+This is because creating an association with a name that clashes with an
+existing method could lead to unintended consequences, such as overriding the
+base method and causing issues with functionality. For example, using names like
+`attributes` or `connection` for associations would be problematic.
-##### `:validate`
+### Updating the Schema
-If you set the `:validate` option to `false`, then new associated objects will not be validated whenever you save this object. By default, this is `true`: new associated objects will be validated when this object is saved.
+Associations are extremely useful, they are responsible for defining the
+relationships between models but they do not update your database schema. You
+are responsible for maintaining your database schema to match your associations.
+This usually involves two main tasks: creating foreign keys for [`belongs_to`
+associations](#belongs-to) and setting up the correct join table for [`has_many
+:through`](#has-many-through) and
+[`has_and_belongs_to_many`](#has-and-belongs-to-many) associations. You can read
+more about when to use a `has_many :through vs has_and_belongs_to_many` [in the
+has many through vs has and belongs to many
+section](#has-many-through-vs-has-and-belongs-to-many).
-#### Scopes for `has_many`
+#### Creating Foreign Keys for `belongs_to` Associations
-There may be times when you wish to customize the query used by `has_many`. Such customizations can be achieved via a scope block. For example:
+When you declare a [`belongs_to` association](#belongs-to), you need to create
+foreign keys as appropriate. For example, consider this model:
```ruby
-class Author < ApplicationRecord
- has_many :books, -> { where processed: true }
+class Book < ApplicationRecord
+ belongs_to :author
end
```
-You can use any of the standard [querying methods](active_record_querying.html) inside the scope block. The following ones are discussed below:
-
-* `where`
-* `extending`
-* `group`
-* `includes`
-* `limit`
-* `offset`
-* `order`
-* `readonly`
-* `select`
-* `distinct`
-
-##### `where`
-
-The `where` method lets you specify the conditions that the associated object must meet.
+This declaration needs to be backed up by a corresponding foreign key column in
+the books table. For a brand new table, the migration might look something like
+this:
```ruby
-class Author < ApplicationRecord
- has_many :confirmed_books, -> { where "confirmed = 1" },
- class_name: "Book"
+class CreateBooks < ActiveRecord::Migration[7.2]
+ def change
+ create_table :books do |t|
+ t.datetime :published_at
+ t.string :book_number
+ t.belongs_to :author
+ end
+ end
end
```
-You can also set conditions via a hash:
+Whereas for an existing table, it might look like this:
```ruby
-class Author < ApplicationRecord
- has_many :confirmed_books, -> { where confirmed: true },
- class_name: "Book"
+class AddAuthorToBooks < ActiveRecord::Migration[7.2]
+ def change
+ add_reference :books, :author
+ end
end
```
-If you use a hash-style `where` option, then record creation via this association will be automatically scoped using the hash. In this case, using `@author.confirmed_books.create` or `@author.confirmed_books.build` will create books where the confirmed column has the value `true`.
-
-##### `extending`
-
-The `extending` method specifies a named module to extend the association proxy. Association extensions are discussed in detail [later in this guide](#association-extensions).
+#### Creating Join Tables for `has_and_belongs_to_many` Associations
-##### `group`
+If you create a `has_and_belongs_to_many` association, you need to explicitly
+create the join table. Unless the name of the join table is explicitly specified
+by using the `:join_table` option, Active Record creates the name by using the
+lexical order of the class names. So a join between author and book models will
+give the default join table name of "authors_books" because "a" outranks "b" in
+lexical ordering.
-The `group` method supplies an attribute name to group the result set by, using a `GROUP BY` clause in the finder SQL.
+Whatever the name, you must manually generate the join table with an appropriate
+migration. For example, consider these associations:
```ruby
-class Author < ApplicationRecord
- has_many :chapters, -> { group 'books.id' },
- through: :books
+class Assembly < ApplicationRecord
+ has_and_belongs_to_many :parts
end
-```
-
-##### `includes`
-
-You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
-```ruby
-class Author < ApplicationRecord
- has_many :books
+class Part < ApplicationRecord
+ has_and_belongs_to_many :assemblies
end
+```
-class Book < ApplicationRecord
- belongs_to :author
- has_many :chapters
-end
+These need to be backed up by a migration to create the `assemblies_parts`
+table.
-class Chapter < ApplicationRecord
- belongs_to :book
-end
+```bash
+$ bin/rails generate migration CreateAssembliesPartsJoinTable assemblies parts
```
-If you frequently retrieve chapters directly from authors (`@author.books.chapters`), then you can make your code somewhat more efficient by including chapters in the association from authors to books:
+You can then fill out the migration and ensure that the table is created without
+a primary key.
```ruby
-class Author < ApplicationRecord
- has_many :books, -> { includes :chapters }
-end
-
-class Book < ApplicationRecord
- belongs_to :author
- has_many :chapters
-end
+class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[7.2]
+ def change
+ create_table :assemblies_parts, id: false do |t|
+ t.bigint :assembly_id
+ t.bigint :part_id
+ end
-class Chapter < ApplicationRecord
- belongs_to :book
+ add_index :assemblies_parts, :assembly_id
+ add_index :assemblies_parts, :part_id
+ end
end
```
-##### `limit`
+We pass `id: false` to `create_table` because the join table does not represent
+a model. If you observe any strange behavior in a `has_and_belongs_to_many`
+association like mangled model IDs, or exceptions about conflicting IDs, chances
+are you forgot to set `id: false` when creating your migration.
-The `limit` method lets you restrict the total number of objects that will be fetched through an association.
+For simplicity, you can also use the method `create_join_table`:
```ruby
-class Author < ApplicationRecord
- has_many :recent_books,
- -> { order('published_at desc').limit(100) },
- class_name: "Book"
+class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[7.2]
+ def change
+ create_join_table :assemblies, :parts do |t|
+ t.index :assembly_id
+ t.index :part_id
+ end
+ end
end
```
-##### `offset`
+You can read more about the `create_join_table` method in the [Active Record
+Migration Guides](active_record_migrations.html#creating-a-join-table)
-The `offset` method lets you specify the starting offset for fetching objects via an association. For example, `-> { offset(11) }` will skip the first 11 records.
+#### Creating Join Tables for `has_many :through` Associations
-##### `order`
-
-The `order` method dictates the order in which associated objects will be received (in the syntax used by an SQL `ORDER BY` clause).
+The main difference in schema implementation between creating a join table for
+`has_many :through` vs `has_and_belongs_to_many` is that the join table for a
+`has_many :through` requires an `id`.
```ruby
-class Author < ApplicationRecord
- has_many :books, -> { order "date_confirmed DESC" }
+class CreateAppointments < ActiveRecord::Migration[7.2]
+ def change
+ create_table :appointments do |t|
+ t.belongs_to :physician
+ t.belongs_to :patient
+ t.datetime :appointment_date
+ t.timestamps
+ end
+ end
end
```
-##### `readonly`
-
-If you use the `readonly` method, then the associated objects will be read-only when retrieved via the association.
-
-##### `select`
-
-The `select` method lets you override the SQL `SELECT` clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.
-
-WARNING: If you specify your own `select`, be sure to include the primary key and foreign key columns of the associated model. If you do not, Rails will throw an error.
-
-##### `distinct`
+### Controlling Association Scope
-Use the `distinct` method to keep the collection free of duplicates. This is
-mostly useful together with the `:through` option.
+By default, associations look for objects only within the current module's
+scope. This feature is particularly useful when declaring Active Record models
+inside a module, as it keeps the associations scoped properly. For example:
```ruby
-class Person < ApplicationRecord
- has_many :readings
- has_many :articles, through: :readings
-end
-```
+module MyApplication
+ module Business
+ class Supplier < ApplicationRecord
+ has_one :account
+ end
-```irb
-irb> person = Person.create(name: 'John')
-irb> article = Article.create(name: 'a1')
-irb> person.articles << article
-irb> person.articles << article
-irb> person.articles.to_a
-=> [#, #]
-irb> Reading.all.to_a
-=> [#, #]
+ class Account < ApplicationRecord
+ belongs_to :supplier
+ end
+ end
+end
```
-In the above case there are two readings and `person.articles` brings out both of
-them even though these records are pointing to the same article.
-
-Now let's set `distinct`:
+In this example, both the `Supplier` and `Account` classes are defined within
+the same module (`MyApplication::Business`). This organization allows you to
+structure your models into folders based on their scope without needing to
+explicitly specify the scope in every association:
```ruby
-class Person
- has_many :readings
- has_many :articles, -> { distinct }, through: :readings
+# app/models/my_application/business/supplier.rb
+module MyApplication
+ module Business
+ class Supplier < ApplicationRecord
+ has_one :account
+ end
+ end
end
```
-```irb
-irb> person = Person.create(name: 'Honda')
-irb> article = Article.create(name: 'a1')
-irb> person.articles << article
-irb> person.articles << article
-irb> person.articles.to_a
-=> [#]
-irb> Reading.all.to_a
-=> [#, #]
+```ruby
+# app/models/my_application/business/account.rb
+module MyApplication
+ module Business
+ class Account < ApplicationRecord
+ belongs_to :supplier
+ end
+ end
+end
```
-In the above case there are still two readings. However `person.articles` shows
-only one article because the collection loads only unique records.
+It is important to note that while model scoping helps organize your code, it
+does not change the naming convention for your database tables. For instance, if
+you have a `MyApplication::Business::Supplier` model, the corresponding database
+table should still follow the naming convention and be named
+`my_application_business_suppliers`.
-If you want to make sure that, upon insertion, all of the records in the
-persisted association are distinct (so that you can be sure that when you
-inspect the association that you will never find duplicate records), you should
-add a unique index on the table itself. For example, if you have a table named
-`readings` and you want to make sure the articles can only be added to a person once,
-you could add the following in a migration:
+However, if the `Supplier` and `Account` models are defined in different scopes,
+the associations will not work by default:
```ruby
-add_index :readings, [:person_id, :article_id], unique: true
-```
-
-Once you have this unique index, attempting to add the article to a person twice
-will raise an `ActiveRecord::RecordNotUnique` error:
+module MyApplication
+ module Business
+ class Supplier < ApplicationRecord
+ has_one :account
+ end
+ end
-```irb
-irb> person = Person.create(name: 'Honda')
-irb> article = Article.create(name: 'a1')
-irb> person.articles << article
-irb> person.articles << article
-ActiveRecord::RecordNotUnique
+ module Billing
+ class Account < ApplicationRecord
+ belongs_to :supplier
+ end
+ end
+end
```
-Note that checking for uniqueness using something like `include?` is subject
-to race conditions. Do not attempt to use `include?` to enforce distinctness
-in an association. For instance, using the article example from above, the
-following code would be racy because multiple users could be attempting this
-at the same time:
+To associate a model with a model in a different namespace, you must specify the
+complete class name in your association declaration:
```ruby
-person.articles << article unless person.articles.include?(article)
-```
-
-#### When are Objects Saved?
-
-When you assign an object to a `has_many` association, that object is automatically saved (in order to update its foreign key). If you assign multiple objects in one statement, then they are all saved.
-
-If any of these saves fails due to validation errors, then the assignment statement returns `false` and the assignment itself is cancelled.
-
-If the parent object (the one declaring the `has_many` association) is unsaved (that is, `new_record?` returns `true`) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.
-
-If you want to assign an object to a `has_many` association without saving the object, use the `collection.build` method.
-
-### `has_and_belongs_to_many` Association Reference
-
-The `has_and_belongs_to_many` association creates a many-to-many relationship with another model. In database terms, this associates two classes via an intermediate join table that includes foreign keys referring to each of the classes.
-
-#### Methods Added by `has_and_belongs_to_many`
-
-When you declare a `has_and_belongs_to_many` association, the declaring class automatically gains several methods related to the association:
-
-* `collection`
-* [`collection<<(object, ...)`][`collection<<`]
-* [`collection.delete(object, ...)`][`collection.delete`]
-* [`collection.destroy(object, ...)`][`collection.destroy`]
-* `collection=(objects)`
-* `collection_singular_ids`
-* `collection_singular_ids=(ids)`
-* [`collection.clear`][]
-* [`collection.empty?`][]
-* [`collection.size`][]
-* [`collection.find(...)`][`collection.find`]
-* [`collection.where(...)`][`collection.where`]
-* [`collection.exists?(...)`][`collection.exists?`]
-* [`collection.build(attributes = {})`][`collection.build`]
-* [`collection.create(attributes = {})`][`collection.create`]
-* [`collection.create!(attributes = {})`][`collection.create!`]
-* [`collection.reload`][]
-
-In all of these methods, `collection` is replaced with the symbol passed as the first argument to `has_and_belongs_to_many`, and `collection_singular` is replaced with the singularized version of that symbol. For example, given the declaration:
+module MyApplication
+ module Business
+ class Supplier < ApplicationRecord
+ has_one :account,
+ class_name: "MyApplication::Billing::Account"
+ end
+ end
-```ruby
-class Part < ApplicationRecord
- has_and_belongs_to_many :assemblies
+ module Billing
+ class Account < ApplicationRecord
+ belongs_to :supplier,
+ class_name: "MyApplication::Business::Supplier"
+ end
+ end
end
```
-Each instance of the `Part` model will have these methods:
+By explicitly declaring the `class_name` option, you can create associations
+across different namespaces, ensuring the correct models are linked regardless
+of their module scope.
-```
-assemblies
-assemblies<<(object, ...)
-assemblies.delete(object, ...)
-assemblies.destroy(object, ...)
-assemblies=(objects)
-assembly_ids
-assembly_ids=(ids)
-assemblies.clear
-assemblies.empty?
-assemblies.size
-assemblies.find(...)
-assemblies.where(...)
-assemblies.exists?(...)
-assemblies.build(attributes = {}, ...)
-assemblies.create(attributes = {})
-assemblies.create!(attributes = {})
-assemblies.reload
-```
+### Bi-directional Associations
+
+In Rails, it's common for associations between models to be bi-directional,
+meaning they need to be declared in both related models. Consider the following
+example:
+
+```ruby
+class Author < ApplicationRecord
+ has_many :books
+end
-##### Additional Column Methods
+class Book < ApplicationRecord
+ belongs_to :author
+end
+```
-If the join table for a `has_and_belongs_to_many` association has additional columns beyond the two foreign keys, these columns will be added as attributes to records retrieved via that association. Records returned with additional attributes will always be read-only, because Rails cannot save changes to those attributes.
+Active Record will attempt to automatically identify that these two models share
+a bi-directional association based on the association name. This information
+allows Active Record to:
-WARNING: The use of extra attributes on the join table in a `has_and_belongs_to_many` association is deprecated. If you require this sort of complex behavior on the table that joins two models in a many-to-many relationship, you should use a `has_many :through` association instead of `has_and_belongs_to_many`.
+* Prevent needless queries for already-loaded data:
-##### `collection`
+ Active Record avoids additional database queries for already-loaded data.
-The `collection` method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation.
+ ```irb
+ irb> author = Author.first
+ irb> author.books.all? do |book|
+ irb> book.author.equal?(author) # No additional queries executed here
+ irb> end
+ => true
+ ```
-```ruby
-@assemblies = @part.assemblies
-```
+* Prevent inconsistent data
-##### `collection<<(object, ...)`
+ Since only one copy of the `Author` object is loaded, it helpes to prevent
+ inconsistencies.
-The [`collection<<`][] method adds one or more objects to the collection by creating records in the join table.
+ ```irb
+ irb> author = Author.first
+ irb> book = author.books.first
+ irb> author.name == book.author.name
+ => true
+ irb> author.name = "Changed Name"
+ irb> author.name == book.author.name
+ => true
+ ```
-```ruby
-@part.assemblies << @assembly1
-```
+* Automatic saving of associations in more cases:
-NOTE: This method is aliased as `collection.concat` and `collection.push`.
+ ```irb
+ irb> author = Author.new
+ irb> book = author.books.new
+ irb> book.save!
+ irb> book.persisted?
+ => true
+ irb> author.persisted?
+ => true
+ ```
-##### `collection.delete(object, ...)`
+* Validate the [presence](active_record_validations.html#presence) and
+ [absence](active_record_validations.html#absence) of associations in more
+ cases:
-The [`collection.delete`][] method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects.
+ ```irb
+ irb> book = Book.new
+ irb> book.valid?
+ => false
+ irb> book.errors.full_messages
+ => ["Author must exist"]
+ irb> author = Author.new
+ irb> book = author.books.new
+ irb> book.valid?
+ => true
+ ```
-```ruby
-@part.assemblies.delete(@assembly1)
-```
+Sometimes, you might need to customize the association with options like
+`:foreign_key` or `:class_name`. When you do this, Rails might not automatically
+recognize the bi-directional association involving `:through` or `:foreign_key`
+options.
-##### `collection.destroy(object, ...)`
+Custom scopes on the opposite association also prevent automatic identification,
+as do custom scopes on the association itself unless
+[`config.active_record.automatic_scope_inversing`][] is set to true.
-The [`collection.destroy`][] method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects.
+For example, consider the following model declarations with a custom foreign
+key:
```ruby
-@part.assemblies.destroy(@assembly1)
+class Author < ApplicationRecord
+ has_many :books
+end
+
+class Book < ApplicationRecord
+ belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
+end
```
-##### `collection=(objects)`
+Due to the `:foreign_key` option, Active Record will not automatically recognize
+the bi-directional association, which can lead to several issues:
-The `collection=` method makes the collection contain only the supplied objects, by adding and deleting as appropriate. The changes are persisted to the database.
+* Execute needless queries for the same data (in this example causing N+1
+ queries):
-##### `collection_singular_ids`
+ ```irb
+ irb> author = Author.first
+ irb> author.books.any? do |book|
+ irb> book.writer.equal?(author) # This executes an author query for every book
+ irb> end
+ => false
+ ```
-The `collection_singular_ids` method returns an array of the ids of the objects in the collection.
+* Reference multiple copies of a model with inconsistent data:
-```ruby
-@assembly_ids = @part.assembly_ids
-```
+ ```irb
+ irb> author = Author.first
+ irb> book = author.books.first
+ irb> author.name == book.writer.name
+ => true
+ irb> author.name = "Changed Name"
+ irb> author.name == book.writer.name
+ => false
+ ```
-##### `collection_singular_ids=(ids)`
+* Fail to autosave associations:
-The `collection_singular_ids=` method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate. The changes are persisted to the database.
+ ```irb
+ irb> author = Author.new
+ irb> book = author.books.new
+ irb> book.save!
+ irb> book.persisted?
+ => true
+ irb> author.persisted?
+ => false
+ ```
-##### `collection.clear`
+* Fail to validate presence or absence:
-The [`collection.clear`][] method removes every object from the collection by deleting the rows from the joining table. This does not destroy the associated objects.
+ ```irb
+ irb> author = Author.new
+ irb> book = author.books.new
+ irb> book.valid?
+ => false
+ irb> book.errors.full_messages
+ => ["Author must exist"]
+ ```
-##### `collection.empty?`
+To resolve these issues, you can explicitly declare bi-directional associations
+using the `:inverse_of` option:
-The [`collection.empty?`][] method returns `true` if the collection does not contain any associated objects.
+```ruby
+class Author < ApplicationRecord
+ has_many :books, inverse_of: 'writer'
+end
-```html+erb
-<% if @part.assemblies.empty? %>
- This part is not used in any assemblies
-<% end %>
+class Book < ApplicationRecord
+ belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
+end
```
-##### `collection.size`
+By including the `:inverse_of` option in the `has_many` association declaration,
+Active Record will recognize the bi-directional association and behave as
+described in the initial examples above.
-The [`collection.size`][] method returns the number of objects in the collection.
+[`config.active_record.automatic_scope_inversing`]:
+ configuring.html#config-active-record-automatic-scope-inversing
-```ruby
-@assembly_count = @part.assemblies.size
-```
+Association References
+----------------------
-##### `collection.find(...)`
+### Options
-The [`collection.find`][] method finds objects within the collection's table.
+While Rails uses intelligent defaults that will work well in most situations,
+there may be times when you want to customize the behavior of the association
+references. Such customizations can be accomplished by passing options blocks
+when you create the association. For example, this association uses two such
+options:
```ruby
-@assembly = @part.assemblies.find(1)
+class Book < ApplicationRecord
+ belongs_to :author, touch: :books_updated_at,
+ counter_cache: true
+end
```
-##### `collection.where(...)`
+Each association supports numerous options which you can read more about in
+[`Options` section of each association in the ActiveRecord Associations
+API](https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html).
+We'll discuss some of the common use cases below.
+
+#### `:class_name`
-The [`collection.where`][] method finds objects within the collection based on the conditions supplied but the objects are loaded lazily meaning that the database is queried only when the object(s) are accessed.
+If the name of the other model cannot be derived from the association name, you
+can use the `:class_name` option to supply the model name. For example, if a
+book belongs to an author, but the actual name of the model containing authors
+is `Patron`, you'd set things up this way:
```ruby
-@new_assemblies = @part.assemblies.where("created_at > ?", 2.days.ago)
+class Book < ApplicationRecord
+ belongs_to :author, class_name: "Patron"
+end
```
-##### `collection.exists?(...)`
+#### `:dependent`
-The [`collection.exists?`][] method checks whether an object meeting the supplied
-conditions exists in the collection's table.
-
-##### `collection.build(attributes = {})`
+Controls what happens to the associated object when its owner is destroyed:
-The [`collection.build`][] method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through the join table will be created, but the associated object will _not_ yet be saved.
+* `:destroy`, when the object is destroyed, `destroy` will be called on its
+ associated objects. This method not only removes the associated records from
+ the database but also ensures that any defined callbacks (like
+ `before_destroy` and `after_destroy`) are executed. This is useful for
+ performing custom logic during the deletion process, such as logging or
+ cleaning up related data.
+* `:delete`, when the object is destroyed, all its associated objects will be
+ deleted directly from the database without calling their `destroy` method.
+ This method performs a direct deletion and bypasses any callbacks or
+ validations in the associated models, making it more efficient but potentially
+ leading to data integrity issues if important cleanup tasks are skipped. Use
+ `delete` when you need to remove records quickly and are confident that no
+ additional actions are required for the associated records.
+* `:destroy_async`: when the object is destroyed, an
+ `ActiveRecord::DestroyAssociationAsyncJob` job is enqueued which will call
+ destroy on its associated objects. Active Job must be set up for this to work.
+ Do not use this option if the association is backed by foreign key constraints
+ in your database. The foreign key constraint actions will occur inside the
+ same transaction that deletes its owner.
+ * `:nullify` causes the foreign key to be set to `NULL`. Polymorphic type
+ column is also nullified on polymorphic associations. Callbacks are not
+ executed.
+ * `:restrict_with_exception` causes an `ActiveRecord::DeleteRestrictionError`
+ exception to be raised if there is an associated record
+ * `:restrict_with_error` causes an error to be added to the owner if there is
+ an associated object
+
+WARNING: You should not specify this option on a `belongs_to` association that
+is connected with a `has_many` association on the other class. Doing so can lead
+to orphaned records in your database because destroying the parent object may
+attempt to destroy its children, which in turn may attempt to destroy the parent
+again, causing inconsistencies.
+
+Do not leave the `:nullify` option for associations with `NOT NULL` database
+constraints. Setting `dependent` to `:destroy` is essential; otherwise, the
+foreign key of the associated object may be set to `NULL`, preventing changes to
+it.
+
+NOTE: The `:dependent` option is ignored with the `:through` option. When using
+`:through`, the join model must have a `belongs_to` association, and the
+deletion affects only the join records, not the associated records.
+
+When using `dependent: :destroy` on a scoped association, only the scoped
+objects are destroyed. For example, in a `Post` model defined as `has_many
+:comments, -> { where published: true }, dependent: :destroy`, calling destroy
+on a post will only delete published comments, leaving unpublished comments
+intact with a foreign key pointing to the deleted post.
+
+You cannot use the `:dependent` option directly on a `has_and_belongs_to_many`
+association. To manage deletions of join table records, handle them manually or
+switch to a `has_many :through` association, which provides more flexibility and
+supports the `:dependent` option.
+
+#### `:foreign_key`
+
+By convention, Rails assumes that the column used to hold the foreign key on
+this model is the name of the association with the suffix `_id` added. The
+`:foreign_key` option lets you set the name of the foreign key directly:
```ruby
-@assembly = @part.assemblies.build({ assembly_name: "Transmission housing" })
+class Supplier < ApplicationRecord
+ has_one :account, foreign_key: "supp_id"
+end
```
-##### `collection.create(attributes = {})`
+NOTE: Rails does not create foreign key columns for you. You need to explicitly
+define them in your migrations.
+
+#### `:primary_key`
-The [`collection.create`][] method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through the join table will be created, and, once it passes all of the validations specified on the associated model, the associated object _will_ be saved.
+By default, Rails uses the `id` column as the primary key for its tables. The
+`:primary_key` option allows you to specify a different column as the primary
+key.
+
+For example, if the `users` table uses `guid` as the primary key instead of
+`id`, and you want the `todos` table to reference `guid` as a foreign key
+(`user_id`), you can configure it like this:
```ruby
-@assembly = @part.assemblies.create({ assembly_name: "Transmission housing" })
+class User < ApplicationRecord
+ self.primary_key = 'guid' # Sets the primary key to guid instead of id
+end
+
+class Todo < ApplicationRecord
+ belongs_to :user, primary_key: 'guid' # References the guid column in users table
+end
```
-##### `collection.create!(attributes = {})`
+When you execute `@user.todos.create`, the `@todo` record will have its
+`user_id` value set to the `guid` value of `@user`.
-Does the same as `collection.create`, but raises `ActiveRecord::RecordInvalid` if the record is invalid.
+`has_and_belongs_to_many` does not support the `:primary_key` option. For this
+type of association, you can achieve similar functionality by using a join table
+with has_many `:through` association, which gives more flexibility and supports
+the `:primary_key` option. You can read more about this in the
+[`has_many :through` section](#has-many-through).
-##### `collection.reload`
+#### `:touch`
-The [`collection.reload`][] method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation.
+If you set the `:touch` option to `true`, then the `updated_at` or `updated_on`
+timestamp on the associated object will be set to the current time whenever this
+object is saved or destroyed:
```ruby
-@assemblies = @part.assemblies.reload
-```
+class Book < ApplicationRecord
+ belongs_to :author, touch: true
+end
-#### Options for `has_and_belongs_to_many`
+class Author < ApplicationRecord
+ has_many :books
+end
+```
-While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the `has_and_belongs_to_many` association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:
+In this case, saving or destroying a book will update the timestamp on the
+associated author. You can also specify a particular timestamp attribute to
+update:
```ruby
-class Parts < ApplicationRecord
- has_and_belongs_to_many :assemblies, -> { readonly },
- autosave: true
+class Book < ApplicationRecord
+ belongs_to :author, touch: :books_updated_at
end
```
-The [`has_and_belongs_to_many`][] association supports these options:
+`has_and_belongs_to_many` does not support the `:touch` option. For this type of
+association, you can achieve similar functionality by using a join table with
+`has_many :through` association. You can read more about this in the
+[`has_many :through` section](#has-many-through).
+
+#### `:validate`
-* `:association_foreign_key`
-* `:autosave`
-* `:class_name`
-* `:foreign_key`
-* `:join_table`
-* `:strict_loading`
-* `:validate`
+If you set the `:validate` option to `true`, then new associated objects will be
+validated whenever you save this object. By default, this is `false`: new
+associated objects will not be validated when this object is saved.
-##### `:association_foreign_key`
+`has_and_belongs_to_many` does not support the `:validate` option. For this type
+of association, you can achieve similar functionality by using a join table with
+has_many `:through` association. You can read more about this in the
+[`has_many :through` section](#has-many-through).
-By convention, Rails assumes that the column in the join table used to hold the foreign key pointing to the other model is the name of that model with the suffix `_id` added. The `:association_foreign_key` option lets you set the name of the foreign key directly:
-TIP: The `:foreign_key` and `:association_foreign_key` options are useful when setting up a many-to-many self-join. For example:
+#### `:inverse_of`
+
+The `:inverse_of` option specifies the name of the `belongs_to` association that
+is the inverse of this association. See the [bi-directional
+association](#bi-directional-associations) section for more details.
```ruby
-class User < ApplicationRecord
- has_and_belongs_to_many :friends,
- class_name: "User",
- foreign_key: "this_user_id",
- association_foreign_key: "other_user_id"
+class Supplier < ApplicationRecord
+ has_one :account, inverse_of: :supplier
end
-```
-
-##### `:autosave`
-If you set the `:autosave` option to `true`, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting `:autosave` to `false` is not the same as not setting the `:autosave` option. If the `:autosave` option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
+class Account < ApplicationRecord
+ belongs_to :supplier, inverse_of: :account
+end
+```
-##### `:class_name`
+#### `:source_type`
-If the name of the other model cannot be derived from the association name, you can use the `:class_name` option to supply the model name. For example, if a part has many assemblies, but the actual name of the model containing assemblies is `Gadget`, you'd set things up this way:
+The `:source_type` option specifies the source association type for a `has_many
+:through` association that proceeds through a [polymorphic
+association](#polymorphic-associations).
```ruby
-class Parts < ApplicationRecord
- has_and_belongs_to_many :assemblies, class_name: "Gadget"
+class Author < ApplicationRecord
+ has_many :books
+ has_many :paperbacks, through: :books, source: :format, source_type: "Paperback"
+end
+
+class Book < ApplicationRecord
+ belongs_to :format, polymorphic: true
end
+
+class Hardback < ApplicationRecord; end
+class Paperback < ApplicationRecord; end
```
-##### `:foreign_key`
+#### `:strict_loading`
+
+Enforces strict loading every time an associated record is loaded through this
+association.
-By convention, Rails assumes that the column in the join table used to hold the foreign key pointing to this model is the name of this model with the suffix `_id` added. The `:foreign_key` option lets you set the name of the foreign key directly:
+#### `:association_foreign_key`
+
+The `:association_foreign_key` can be found on a `has_and_belongs_to_many`
+relationship. By convention, Rails assumes that the column in the join table
+used to hold the foreign key pointing to the other model is the name of that
+model with the suffix `_id` added. The `:association_foreign_key` option lets
+you set the name of the foreign key directly For example:
```ruby
class User < ApplicationRecord
@@ -2647,21 +2784,20 @@ class User < ApplicationRecord
end
```
-##### `:join_table`
-
-If the default name of the join table, based on lexical ordering, is not what you want, you can use the `:join_table` option to override the default.
+TIP: The `:foreign_key` and `:association_foreign_key` options are useful when
+setting up a many-to-many self-join.
-##### `:strict_loading`
+#### `:join_table`
-Enforces strict loading every time an associated record is loaded through this association.
+The `:join_table` can be found on a `has_and_belongs_to_many` relationship. If
+the default name of the join table, based on lexical ordering, is not what you
+want, you can use the `:join_table` option to override the default.
-##### `:validate`
+### Scopes
-If you set the `:validate` option to `false`, then new associated objects will not be validated whenever you save this object. By default, this is `true`: new associated objects will be validated when this object is saved.
-
-#### Scopes for `has_and_belongs_to_many`
-
-There may be times when you wish to customize the query used by `has_and_belongs_to_many`. Such customizations can be achieved via a scope block. For example:
+Scopes allow you to specify common queries that can be referenced as method
+calls on the association objects. This is useful for defining custom queries
+that are reused in multiple places in your application. For example:
```ruby
class Parts < ApplicationRecord
@@ -2669,22 +2805,21 @@ class Parts < ApplicationRecord
end
```
-You can use any of the standard [querying methods](active_record_querying.html) inside the scope block. The following ones are discussed below:
+#### General Scopes
+
+You can use any of the standard [querying methods](active_record_querying.html)
+inside the scope block. The following ones are discussed below:
+
* `where`
-* `extending`
-* `group`
* `includes`
-* `limit`
-* `offset`
-* `order`
* `readonly`
* `select`
-* `distinct`
##### `where`
-The `where` method lets you specify the conditions that the associated object must meet.
+The `where` method lets you specify the conditions that the associated object
+must meet.
```ruby
class Parts < ApplicationRecord
@@ -2702,401 +2837,471 @@ class Parts < ApplicationRecord
end
```
-If you use a hash-style `where`, then record creation via this association will be automatically scoped using the hash. In this case, using `@parts.assemblies.create` or `@parts.assemblies.build` will create assemblies where the `factory` column has the value "Seattle".
+If you use a hash-style `where`, then record creation via this association will
+be automatically scoped using the hash. In this case, using
+`@parts.assemblies.create` or `@parts.assemblies.build` will create assemblies
+where the `factory` column has the value "Seattle".
-##### `extending`
-
-The `extending` method specifies a named module to extend the association proxy. Association extensions are discussed in detail [later in this guide](#association-extensions).
-
-##### `group`
+##### `includes`
-The `group` method supplies an attribute name to group the result set by, using a `GROUP BY` clause in the finder SQL.
+You can use the `includes` method to specify second-order associations that
+should be eager-loaded when this association is used. For example, consider
+these models:
```ruby
-class Parts < ApplicationRecord
- has_and_belongs_to_many :assemblies, -> { group "factory" }
+class Supplier < ApplicationRecord
+ has_one :account
end
-```
-
-##### `includes`
-
-You can use the `includes` method to specify second-order associations that should be eager-loaded when this association is used.
-##### `limit`
-
-The `limit` method lets you restrict the total number of objects that will be fetched through an association.
+class Account < ApplicationRecord
+ belongs_to :supplier
+ belongs_to :representative
+end
-```ruby
-class Parts < ApplicationRecord
- has_and_belongs_to_many :assemblies,
- -> { order("created_at DESC").limit(50) }
+class Representative < ApplicationRecord
+ has_many :accounts
end
```
-##### `offset`
-
-The `offset` method lets you specify the starting offset for fetching objects via an association. For example, if you set `offset(11)`, it will skip the first 11 records.
-
-##### `order`
-
-The `order` method dictates the order in which associated objects will be received (in the syntax used by an SQL `ORDER BY` clause).
+If you frequently retrieve representatives directly from suppliers
+(`@supplier.account.representative`), then you can make your code somewhat more
+efficient by including representatives in the association from suppliers to
+accounts:
```ruby
-class Parts < ApplicationRecord
- has_and_belongs_to_many :assemblies,
- -> { order "assembly_name ASC" }
+class Supplier < ApplicationRecord
+ has_one :account, -> { includes :representative }
end
-```
-
-##### `readonly`
-
-If you use the `readonly` method, then the associated objects will be read-only when retrieved via the association.
-
-##### `select`
-
-The `select` method lets you override the SQL `SELECT` clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.
-
-##### `distinct`
-
-Use the `distinct` method to remove duplicates from the collection.
-
-#### When are Objects Saved?
-
-When you assign an object to a `has_and_belongs_to_many` association, that object is automatically saved (in order to update the join table). If you assign multiple objects in one statement, then they are all saved.
-If any of these saves fails due to validation errors, then the assignment statement returns `false` and the assignment itself is cancelled.
-
-If the parent object (the one declaring the `has_and_belongs_to_many` association) is unsaved (that is, `new_record?` returns `true`) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.
+class Account < ApplicationRecord
+ belongs_to :supplier
+ belongs_to :representative
+end
-If you want to assign an object to a `has_and_belongs_to_many` association without saving the object, use the `collection.build` method.
+class Representative < ApplicationRecord
+ has_many :accounts
+end
+```
-### Association Callbacks
+NOTE: There's no need to use `includes` for immediate associations - that is, if
+you have `Book belongs_to :author`, then the author is eager-loaded
+automatically when it's needed.
-Normal callbacks hook into the life cycle of Active Record objects, allowing you to work with those objects at various points. For example, you can use a `:before_save` callback to cause something to happen just before an object is saved.
+##### `readonly`
-Association callbacks are similar to normal callbacks, but they are triggered by events in the life cycle of a collection. There are four available association callbacks:
+If you use `readonly`, then the associated object will be read-only when
+retrieved via the association.
-* `before_add`
-* `after_add`
-* `before_remove`
-* `after_remove`
+```ruby
+class Book < ApplicationRecord
+ belongs_to :author, -> { readonly }
+end
+```
-You define association callbacks by adding options to the association declaration. For example:
+This is useful when you want to prevent the associated object from being
+modified through the association. For example, if you have a `Book` model that
+`belongs_to :author`, you can use `readonly` to prevent the author from being
+modified through the book:
```ruby
-class Author < ApplicationRecord
- has_many :books, before_add: :check_credit_limit
-
- def check_credit_limit(book)
- # ...
- end
-end
+@book.author = Author.first
+@book.author.save! # This will raise an ActiveRecord::ReadOnlyRecord error
```
-Read more about association callbacks in the [Active Record Callbacks Guide](active_record_callbacks.html#association-callbacks)
+##### `select`
-### Association Extensions
+The `select` method lets you override the SQL `SELECT` clause used to retrieve
+data about the associated object. By default, Rails retrieves all columns.
-You're not limited to the functionality that Rails automatically builds into association proxy objects. You can also extend these objects through anonymous modules, adding new finders, creators, or other methods. For example:
+For example, if you have an `Author` model with many `Book`s, but you only want
+to retrieve the `title` of each book:
```ruby
class Author < ApplicationRecord
- has_many :books do
- def find_by_book_prefix(book_number)
- find_by(category_id: book_number[0..2])
- end
- end
+ has_many :books, -> { select(:id, :title) } # Only select id and title columns
+end
+
+class Book < ApplicationRecord
+ belongs_to :author
end
```
-If you have an extension that should be shared by many associations, you can use a named extension module. For example:
+Now, when you access an author's books, only the `id` and `title` columns will
+be retrieved from the `books` table.
+
+TIP: If you use the `select` method on a `belongs_to` association, you should
+also set the `:foreign_key` option to guarantee correct results. For example:
```ruby
-module FindRecentExtension
- def find_recent
- where("created_at > ?", 5.days.ago)
- end
+class Book < ApplicationRecord
+ belongs_to :author, -> { select(:id, :name) }, foreign_key: 'author_id' # Only select id and name columns
end
class Author < ApplicationRecord
- has_many :books, -> { extending FindRecentExtension }
-end
-
-class Supplier < ApplicationRecord
- has_many :deliveries, -> { extending FindRecentExtension }
+ has_many :books
end
```
-Extensions can refer to the internals of the association proxy using these three attributes of the `proxy_association` accessor:
+In this case, when you access a book's author, only the `id` and `name` columns
+will be retrieved from the `authors` table.
-* `proxy_association.owner` returns the object that the association is a part of.
-* `proxy_association.reflection` returns the reflection object that describes the association.
-* `proxy_association.target` returns the associated object for `belongs_to` or `has_one`, or the collection of associated objects for `has_many` or `has_and_belongs_to_many`.
+#### Collection Scopes
-### Association Scoping using the Association Owner
+`has_many` and `has_and_belongs_to_many` are associations that deal with
+collections of records, so you can use additional methods like `group`, `limit`,
+`order`, `select`, and `distinct` to customize the query used by the
+association.
+
+##### `group`
-The owner of the association can be passed as a single argument to the scope
-block in situations where you need even more control over the association
-scope. However, as a caveat, preloading the association will no longer be
-possible.
+The `group` method supplies an attribute name to group the result set by, using
+a `GROUP BY` clause in the finder SQL.
```ruby
-class Supplier < ApplicationRecord
- has_one :account, ->(supplier) { where active: supplier.active? }
+class Parts < ApplicationRecord
+ has_and_belongs_to_many :assemblies, -> { group "factory" }
end
```
-Single Table Inheritance (STI)
-------------------------------
+##### `limit`
-Sometimes, you may want to share fields and behavior between different models.
-Let's say we have Car, Motorcycle, and Bicycle models. We will want to share
-the `color` and `price` fields and some methods for all of them, but having some
-specific behavior for each, and separated controllers too.
+The `limit` method lets you restrict the total number of objects that will be
+fetched through an association.
-First, let's generate the base Vehicle model:
+```ruby
+class Parts < ApplicationRecord
+ has_and_belongs_to_many :assemblies,
+ -> { order("created_at DESC").limit(50) }
+end
+```
-```bash
-$ bin/rails generate model vehicle type:string color:string price:decimal{10.2}
+##### `order`
+
+The `order` method dictates the order in which associated objects will be
+received (in the syntax used by an SQL `ORDER BY` clause).
+
+```ruby
+class Author < ApplicationRecord
+ has_many :books, -> { order "date_confirmed DESC" }
+end
```
-Did you note we are adding a "type" field? Since all models will be saved in a
-single database table, Rails will save in this column the name of the model that
-is being saved. In our example, this can be "Car", "Motorcycle" or "Bicycle."
-STI won't work without a "type" field in the table.
+##### `select`
-Next, we will generate the Car model that inherits from Vehicle. For this,
-we can use the `--parent=PARENT` option, which will generate a model that
-inherits from the specified parent and without equivalent migration (since the
-table already exists).
+The `select` method lets you override the SQL `SELECT` clause that is used to
+retrieve data about the associated objects. By default, Rails retrieves all
+columns.
-For example, to generate the Car model:
+WARNING: If you specify your own `select`, be sure to include the primary key
+and foreign key columns of the associated model. If you do not, Rails will throw
+an error.
-```bash
-$ bin/rails generate model car --parent=Vehicle
-```
+##### `distinct`
-The generated model will look like this:
+Use the `distinct` method to keep the collection free of duplicates. This is
+mostly useful together with the `:through` option.
```ruby
-class Car < Vehicle
+class Person < ApplicationRecord
+ has_many :readings
+ has_many :articles, through: :readings
end
```
-This means that all behavior added to Vehicle is available for Car too, as
-associations, public methods, etc.
+```irb
+irb> person = Person.create(name: 'John')
+irb> article = Article.create(name: 'a1')
+irb> person.articles << article
+irb> person.articles << article
+irb> person.articles.to_a
+=> [#, #]
+irb> Reading.all.to_a
+=> [#, #]
+```
-Creating a car will save it in the `vehicles` table with "Car" as the `type` field:
+In the above case there are two readings and `person.articles` brings out both
+of them even though these records are pointing to the same article.
+
+Now let's set `distinct`:
```ruby
-Car.create(color: 'Red', price: 10000)
+class Person
+ has_many :readings
+ has_many :articles, -> { distinct }, through: :readings
+end
```
-will generate the following SQL:
-
-```sql
-INSERT INTO "vehicles" ("type", "color", "price") VALUES ('Car', 'Red', 10000)
+```irb
+irb> person = Person.create(name: 'Honda')
+irb> article = Article.create(name: 'a1')
+irb> person.articles << article
+irb> person.articles << article
+irb> person.articles.to_a
+=> [#]
+irb> Reading.all.to_a
+=> [#, #]
```
-Querying car records will search only for vehicles that are cars:
+In the above case there are still two readings. However `person.articles` shows
+only one article because the collection loads only unique records.
+
+If you want to make sure that, upon insertion, all of the records in the
+persisted association are distinct (so that you can be sure that when you
+inspect the association that you will never find duplicate records), you should
+add a unique index on the table itself. For example, if you have a table named
+`readings` and you want to make sure the articles can only be added to a person
+once, you could add the following in a migration:
```ruby
-Car.all
+add_index :readings, [:person_id, :article_id], unique: true
```
-will run a query like:
+Once you have this unique index, attempting to add the article to a person twice
+will raise an `ActiveRecord::RecordNotUnique` error:
-```sql
-SELECT "vehicles".* FROM "vehicles" WHERE "vehicles"."type" IN ('Car')
+```irb
+irb> person = Person.create(name: 'Honda')
+irb> article = Article.create(name: 'a1')
+irb> person.articles << article
+irb> person.articles << article
+ActiveRecord::RecordNotUnique
```
-### Overriding the inheritance column
-
-There may be cases (like when working with a legacy database) where you need to
-override the name of the inheritance column. This can be achieved with the
-[inheritance_column][] method.
+Note that checking for uniqueness using something like `include?` is subject to
+race conditions. Do not attempt to use `include?` to enforce distinctness in an
+association. For instance, using the article example from above, the following
+code would be racy because multiple users could be attempting this at the same
+time:
```ruby
-# Schema: vehicles[ id, kind, created_at, updated_at ]
-class Vehicle < ApplicationRecord
- self.inheritance_column = "kind"
-end
+person.articles << article unless person.articles.include?(article)
+```
-class Car < Vehicle
-end
+#### Using the Association Owner
-Car.create
-# => #
+You can pass the owner of the association as a single argument to the scope
+block for even more control over the association scope. However, be aware that
+doing this will make preloading the association impossible.
+
+For example:
+
+```ruby
+class Supplier < ApplicationRecord
+ has_one :account, ->(supplier) { where active: supplier.active? }
+end
```
-### Disabling the inheritance column
+In this example, the `account` association of the `Supplier` model is scoped
+based on the `active` status of the supplier.
-There may be cases (like when working with a legacy database) where you need to
-disable Single Table Inheritance altogether. Otherwise, you'll raise
-[`ActiveRecord::SubclassNotFound`][].
+By utilizing association extensions and scoping with the association owner, you
+can create more dynamic and context-aware associations in your Rails
+applications.
-This can be achieved by setting the [inheritance_column][] to `nil`.
+### Counter Cache
+
+The `:counter_cache` option in Rails helps improve the efficiency of finding the
+number of associated objects. Consider the following models:
```ruby
-# Schema: vehicles[ id, type, created_at, updated_at ]
-class Vehicle < ApplicationRecord
- self.inheritance_column = nil
+class Book < ApplicationRecord
+ belongs_to :author
end
-Vehicle.create!(type: "Car")
-# => #
+class Author < ApplicationRecord
+ has_many :books
+end
```
-[inheritance_column]: https://api.rubyonrails.org/classes/ActiveRecord/ModelSchema.html#method-c-inheritance_column
-[`ActiveRecord::SubclassNotFound`]: https://api.rubyonrails.org/classes/ActiveRecord/SubclassNotFound.html
+By default, querying `@auth books.size` results in a database call to perform a
+`COUNT(*)` query. To optimize this, you can add a counter cache to the
+_belonging_ model (in this case, `Book`). This way, Rails can return the count
+directly from the cache without querying the database.
-Delegated Types
-----------------
+```ruby
+class Book < ApplicationRecord
+ belongs_to :author, counter_cache: true
+end
-[`Single Table Inheritance (STI)`](#single-table-inheritance-sti) works best when there is little difference between subclasses and their attributes, but includes all attributes of all subclasses you need to create a single table.
+class Author < ApplicationRecord
+ has_many :books
+end
+```
-The disadvantage of this approach is that it results in bloat to that table. Since it will even include attributes specific to a subclass that aren't used by anything else.
+With this declaration, Rails will keep the cache value up to date, and then
+return that value in response to the `size` method, avoiding the database call.
-In the following example, there are two Active Record models that inherit from the same "Entry" class which includes the `subject` attribute.
+Although the `:counter_cache` option is specified on the model with the
+`belongs_to` declaration, the actual column must be added to the _associated_
+(in this case `has_many`) model. In this example, you need to add a
+`books_count` column to the `Author` model:
```ruby
-# Schema: entries[ id, type, subject, created_at, updated_at]
-class Entry < ApplicationRecord
+class AddBooksCountToAuthors < ActiveRecord::Migration[6.0]
+ def change
+ add_column :authors, :books_count, :integer, default: 0, null: false
+ end
end
+```
-class Comment < Entry
+You can specify a custom column name in the `counter_cache` declaration instead
+of using the default `books_count`. For example, to use `count_of_books`:
+
+```ruby
+class Book < ApplicationRecord
+ belongs_to :author, counter_cache: :count_of_books
end
-class Message < Entry
+class Author < ApplicationRecord
+ has_many :books
end
```
-Delegated types solves this problem, via `delegated_type`.
+NOTE: You only need to specify the `:counter_cache` option on the `belongs_to`
+side of the association.
-In order to use delegated types, we have to model our data in a particular way. The requirements are as follows:
+Using counter caches on existing large tables can be troublesome. To avoid
+locking the table for too long, the column values must be backfilled separately
+from the column addition. This backfill must also happen before using
+`:counter_cache`; otherwise, methods like `size`, `any?`, etc., which rely on
+counter caches, may return incorrect results.
-* There is a superclass that stores shared attributes among all subclasses in its table.
-* Each subclass must inherit from the super class, and will have a separate table for any additional attributes specific to it.
+To backfill values safely while keeping counter cache columns updated with child
+record creation/removal and ensuring methods always get results from the
+database (avoiding potentially incorrect counter cache values), use
+`counter_cache: { active: false }`. This setting ensures that methods always
+fetch results from the database, avoiding incorrect values from an uninitialized
+counter cache. If you need to specify a custom column name, use `counter_cache:
+{ active: false, column: :my_custom_counter }`.
-This eliminates the need to define attributes in a single table that are unintentionally shared among all subclasses.
+If for some reason you change the value of an owner model's primary key, and do
+not also update the foreign keys of the counted models, then the counter cache
+may have stale data. In other words, any orphaned models will still count
+towards the counter. To fix a stale counter cache, use
+[`reset_counters`](https://api.rubyonrails.org/classes/ActiveRecord/CounterCache/ClassMethods.html#method-i-reset_counters).
-In order to apply this to our example above, we need to regenerate our models.
-First, let's generate the base `Entry` model which will act as our superclass:
+### Callbacks
-```bash
-$ bin/rails generate model entry entryable_type:string entryable_id:integer
-```
+[Normal callbacks](active_record_callbacks.html) hook into the life cycle of
+Active Record objects, allowing you to work with those objects at various
+points. For example, you can use a `:before_save` callback to cause something to
+happen just before an object is saved.
-Then, we will generate new `Message` and `Comment` models for delegation:
+Association callbacks are similar to normal callbacks, but they are triggered by
+events in the life cycle of a collection associated with an Active Record
+object. There are four available association callbacks:
-```bash
-$ bin/rails generate model message subject:string body:string
-$ bin/rails generate model comment content:string
-```
+* `before_add`
+* `after_add`
+* `before_remove`
+* `after_remove`
-After running the generators, we should end up with models that look like this:
+You define association callbacks by adding options to the association
+declaration. For example:
```ruby
-# Schema: entries[ id, entryable_type, entryable_id, created_at, updated_at ]
-class Entry < ApplicationRecord
-end
-
-# Schema: messages[ id, subject, body, created_at, updated_at ]
-class Message < ApplicationRecord
-end
+class Author < ApplicationRecord
+ has_many :books, before_add: :check_credit_limit
-# Schema: comments[ id, content, created_at, updated_at ]
-class Comment < ApplicationRecord
+ def check_credit_limit(book)
+ throw(:abort) if limit_reached?
+ end
end
```
-### Declare `delegated_type`
+In this example, the `Author` model has a `has_many` association with `books`.
+The `before_add` callback `check_credit_limit` is triggered before a book is
+added to the collection. If the `limit_reached?` method returns `true`, the book
+is not added to the collection.
-First, declare a `delegated_type` in the superclass `Entry`.
+By using these association callbacks, you can customize the behavior of your
+associations, ensuring that specific actions are taken at key points in the life
+cycle of your collections.
-```ruby
-class Entry < ApplicationRecord
- delegated_type :entryable, types: %w[ Message Comment ], dependent: :destroy
-end
-```
+Read more about association callbacks in the [Active Record Callbacks
+Guide](active_record_callbacks.html#association-callbacks)
-The `entryable` parameter specifies the field to use for delegation, and include the types `Message` and `Comment` as the delegate classes.
+### Extensions
-The `Entry` class has `entryable_type` and `entryable_id` fields. This is the field with the `_type`, `_id` suffixes added to the name `entryable` in the `delegated_type` definition.
-`entryable_type` stores the subclass name of the delegatee, and `entryable_id` stores the record id of the delegatee subclass.
+Rails provides the ability to extend the functionality of association proxy
+objects, which manage associations, by adding new finders, creators, or other
+methods through anonymous modules. This feature allows you to customize
+associations to meet the specific needs of your application.
-Next, we must define a module to implement those delegated types, by declaring the `as: :entryable` parameter to the `has_one` association.
+You can extend a `has_many` association with custom methods directly within the
+model definition. For example:
```ruby
-module Entryable
- extend ActiveSupport::Concern
-
- included do
- has_one :entry, as: :entryable, touch: true
+class Author < ApplicationRecord
+ has_many :books do
+ def find_by_book_prefix(book_number)
+ find_by(category_id: book_number[0..2])
+ end
end
end
```
-And then include the created module in your subclass.
+In this example, the `find_by_book_prefix` method is added to the `books`
+association of the `Author` model. This custom method allows you to find `books`
+based on a specific prefix of the `book_number`.
+
+If you have an extension that should be shared by multiple associations, you can
+use a named extension module. For example:
```ruby
-class Message < ApplicationRecord
- include Entryable
+module FindRecentExtension
+ def find_recent
+ where("created_at > ?", 5.days.ago)
+ end
end
-class Comment < ApplicationRecord
- include Entryable
+class Author < ApplicationRecord
+ has_many :books, -> { extending FindRecentExtension }
end
-```
-With this definition complete, our `Entry` delegator now provides the following methods:
-
-| Method | Return |
-|---|---|
-| `Entry.entryable_types` | ["Message", "Comment"] |
-| `Entry#entryable_class` | Message or Comment |
-| `Entry#entryable_name` | "message" or "comment" |
-| `Entry.messages` | `Entry.where(entryable_type: "Message")` |
-| `Entry#message?` | Returns true when `entryable_type == "Message"` |
-| `Entry#message` | Returns the message record, when `entryable_type == "Message"`, otherwise `nil` |
-| `Entry#message_id` | Returns `entryable_id`, when `entryable_type == "Message"`, otherwise `nil` |
-| `Entry.comments` | `Entry.where(entryable_type: "Comment")` |
-| `Entry#comment?` | Returns true when `entryable_type == "Comment"` |
-| `Entry#comment` | Returns the comment record, when `entryable_type == "Comment"`, otherwise `nil` |
-| `Entry#comment_id` | Returns entryable_id, when `entryable_type == "Comment"`, otherwise `nil` |
+class Supplier < ApplicationRecord
+ has_many :deliveries, -> { extending FindRecentExtension }
+end
+```
-### Object creation
+In this case, the `FindRecentExtension` module is used to add a `find_recent`
+method to both the `books` association in the `Author` model and the
+`deliveries` association in the `Supplier` model. This method retrieves records
+created within the last five days.
-When creating a new `Entry` object, we can specify the `entryable` subclass at the same time.
+Extensions can interact with the internals of the association proxy using the
+`proxy_association` accessor. The `proxy_association` provides three important
+attributes:
-```ruby
-Entry.create! entryable: Message.new(subject: "hello!")
-```
+* `proxy_association.owner` returns the object that the association is a part
+ of.
+* `proxy_association.reflection` returns the reflection object that describes
+ the association.
+* `proxy_association.target` returns the associated object for `belongs_to` or
+ `has_one`, or the collection of associated objects for `has_many` or
+ `has_and_belongs_to_many`.
-### Adding further delegation
+These attributes allow extensions to access and manipulate the association
+proxy's internal state and behavior.
-We can expand our `Entry` delegator and enhance it further by defining `delegate` and using polymorphism on the subclasses.
-For example, to delegate the `title` method from `Entry` to it's subclasses:
+Here's an advanced example demonstrating how to use these attributes in an
+extension:
```ruby
-class Entry < ApplicationRecord
- delegated_type :entryable, types: %w[ Message Comment ]
- delegate :title, to: :entryable
-end
-
-class Message < ApplicationRecord
- include Entryable
-
- def title
- subject
+module AdvancedExtension
+ def find_and_log(query)
+ results = where(query)
+ proxy_association.owner.logger.info("Querying #{proxy_association.reflection.name} with #{query}")
+ results
end
end
-class Comment < ApplicationRecord
- include Entryable
-
- def title
- content.truncate(20)
- end
+class Author < ApplicationRecord
+ has_many :books, -> { extending AdvancedExtension }
end
```
+
+In this example, the `find_and_log` method performs a query on the association
+and logs the query details using the owner's logger. The method accesses the
+owner's logger via `proxy_association.owner` and the association's name via
+`proxy_association.reflection`.name.